RFC 5661
Network File System (NFS) Version 4 Minor Version 1 Protocol
4. Filehandles
The filehandle in the NFS protocol is a per-server unique identifier for a file system object. The contents of the filehandle are opaque to the client. Therefore, the server is responsible for translating the filehandle to an internal representation of the file system object.
4.1. Obtaining the First Filehandle
The operations of the NFS protocol are defined in terms of one or more filehandles. Therefore, the client needs a filehandle to initiate communication with the server. With the NFSv3 protocol (RFC 1813 [31]), there exists an ancillary protocol to obtain this first filehandle. The MOUNT protocol, RPC program number 100005, provides the mechanism of translating a string-based file system pathname to a filehandle, which can then be used by the NFS protocols. The MOUNT protocol has deficiencies in the area of security and use via firewalls. This is one reason that the use of the public filehandle was introduced in RFC 2054 [42] and RFC 2055 [43]. With the use of the public filehandle in combination with the LOOKUP operation in the NFSv3 protocol, it has been demonstrated that the MOUNT protocol is unnecessary for viable interaction between NFS client and server. Therefore, the NFSv4.1 protocol will not use an ancillary protocol for translation from string-based pathnames to a filehandle. Two special filehandles will be used as starting points for the NFS client.4.1.1. Root Filehandle
The first of the special filehandles is the ROOT filehandle. The ROOT filehandle is the "conceptual" root of the file system namespace at the NFS server. The client uses or starts with the ROOT filehandle by employing the PUTROOTFH operation. The PUTROOTFH operation instructs the server to set the "current" filehandle to the ROOT of the server's file tree. Once this PUTROOTFH operation is used, the client can then traverse the entirety of the server's file tree with the LOOKUP operation. A complete discussion of the server namespace is in Section 7.4.1.2. Public Filehandle
The second special filehandle is the PUBLIC filehandle. Unlike the ROOT filehandle, the PUBLIC filehandle may be bound or represent an arbitrary file system object at the server. The server is responsible for this binding. It may be that the PUBLIC filehandle and the ROOT filehandle refer to the same file system object. However, it is up to the administrative software at the server and the policies of the server administrator to define the binding of the PUBLIC filehandle and server file system object. The client may not make any assumptions about this binding. The client uses the PUBLIC filehandle via the PUTPUBFH operation.
4.2. Filehandle Types
In the NFSv3 protocol, there was one type of filehandle with a single set of semantics. This type of filehandle is termed "persistent" in NFSv4.1. The semantics of a persistent filehandle remain the same as before. A new type of filehandle introduced in NFSv4.1 is the "volatile" filehandle, which attempts to accommodate certain server environments. The volatile filehandle type was introduced to address server functionality or implementation issues that make correct implementation of a persistent filehandle infeasible. Some server environments do not provide a file-system-level invariant that can be used to construct a persistent filehandle. The underlying server file system may not provide the invariant or the server's file system programming interfaces may not provide access to the needed invariant. Volatile filehandles may ease the implementation of server functionality such as hierarchical storage management or file system reorganization or migration. However, the volatile filehandle increases the implementation burden for the client. Since the client will need to handle persistent and volatile filehandles differently, a file attribute is defined that may be used by the client to determine the filehandle types being returned by the server.4.2.1. General Properties of a Filehandle
The filehandle contains all the information the server needs to distinguish an individual file. To the client, the filehandle is opaque. The client stores filehandles for use in a later request and can compare two filehandles from the same server for equality by doing a byte-by-byte comparison. However, the client MUST NOT otherwise interpret the contents of filehandles. If two filehandles from the same server are equal, they MUST refer to the same file. Servers SHOULD try to maintain a one-to-one correspondence between filehandles and files, but this is not required. Clients MUST use filehandle comparisons only to improve performance, not for correct behavior. All clients need to be prepared for situations in which it cannot be determined whether two filehandles denote the same object and in such cases, avoid making invalid assumptions that might cause incorrect behavior. Further discussion of filehandle and attribute comparison in the context of data caching is presented in Section 10.3.4. As an example, in the case that two different pathnames when traversed at the server terminate at the same file system object, the server SHOULD return the same filehandle for each path. This can
occur if a hard link (see [6]) is used to create two file names that refer to the same underlying file object and associated data. For example, if paths /a/b/c and /a/d/c refer to the same file, the server SHOULD return the same filehandle for both pathnames' traversals.4.2.2. Persistent Filehandle
A persistent filehandle is defined as having a fixed value for the lifetime of the file system object to which it refers. Once the server creates the filehandle for a file system object, the server MUST accept the same filehandle for the object for the lifetime of the object. If the server restarts, the NFS server MUST honor the same filehandle value as it did in the server's previous instantiation. Similarly, if the file system is migrated, the new NFS server MUST honor the same filehandle as the old NFS server. The persistent filehandle will be become stale or invalid when the file system object is removed. When the server is presented with a persistent filehandle that refers to a deleted object, it MUST return an error of NFS4ERR_STALE. A filehandle may become stale when the file system containing the object is no longer available. The file system may become unavailable if it exists on removable media and the media is no longer available at the server or the file system in whole has been destroyed or the file system has simply been removed from the server's namespace (i.e., unmounted in a UNIX environment).4.2.3. Volatile Filehandle
A volatile filehandle does not share the same longevity characteristics of a persistent filehandle. The server may determine that a volatile filehandle is no longer valid at many different points in time. If the server can definitively determine that a volatile filehandle refers to an object that has been removed, the server should return NFS4ERR_STALE to the client (as is the case for persistent filehandles). In all other cases where the server determines that a volatile filehandle can no longer be used, it should return an error of NFS4ERR_FHEXPIRED. The REQUIRED attribute "fh_expire_type" is used by the client to determine what type of filehandle the server is providing for a particular file system. This attribute is a bitmask with the following values:
FH4_PERSISTENT The value of FH4_PERSISTENT is used to indicate a
persistent filehandle, which is valid until the object is removed
from the file system. The server will not return
NFS4ERR_FHEXPIRED for this filehandle. FH4_PERSISTENT is defined
as a value in which none of the bits specified below are set.
FH4_VOLATILE_ANY The filehandle may expire at any time, except as
specifically excluded (i.e., FH4_NO_EXPIRE_WITH_OPEN).
FH4_NOEXPIRE_WITH_OPEN May only be set when FH4_VOLATILE_ANY is set.
If this bit is set, then the meaning of FH4_VOLATILE_ANY is
qualified to exclude any expiration of the filehandle when it is
open.
FH4_VOL_MIGRATION The filehandle will expire as a result of a file
system transition (migration or replication), in those cases in
which the continuity of filehandle use is not specified by handle
class information within the fs_locations_info attribute. When
this bit is set, clients without access to fs_locations_info
information should assume that filehandles will expire on file
system transitions.
FH4_VOL_RENAME The filehandle will expire during rename. This
includes a rename by the requesting client or a rename by any
other client. If FH4_VOL_ANY is set, FH4_VOL_RENAME is redundant.
Servers that provide volatile filehandles that can expire while open
require special care as regards handling of RENAMEs and REMOVEs.
This situation can arise if FH4_VOL_MIGRATION or FH4_VOL_RENAME is
set, if FH4_VOLATILE_ANY is set and FH4_NOEXPIRE_WITH_OPEN is not
set, or if a non-read-only file system has a transition target in a
different handle class. In these cases, the server should deny a
RENAME or REMOVE that would affect an OPEN file of any of the
components leading to the OPEN file. In addition, the server should
deny all RENAME or REMOVE requests during the grace period, in order
to make sure that reclaims of files where filehandles may have
expired do not do a reclaim for the wrong file.
Volatile filehandles are especially suitable for implementation of
the pseudo file systems used to bridge exports. See Section 7.5 for
a discussion of this.
4.3. One Method of Constructing a Volatile Filehandle
A volatile filehandle, while opaque to the client, could contain:
[volatile bit = 1 | server boot time | slot | generation number]
o slot is an index in the server volatile filehandle table
o generation number is the generation number for the table entry/
slot
When the client presents a volatile filehandle, the server makes the
following checks, which assume that the check for the volatile bit
has passed. If the server boot time is less than the current server
boot time, return NFS4ERR_FHEXPIRED. If slot is out of range, return
NFS4ERR_BADHANDLE. If the generation number does not match, return
NFS4ERR_FHEXPIRED.
When the server restarts, the table is gone (it is volatile).
If the volatile bit is 0, then it is a persistent filehandle with a
different structure following it.
4.4. Client Recovery from Filehandle Expiration
If possible, the client SHOULD recover from the receipt of an
NFS4ERR_FHEXPIRED error. The client must take on additional
responsibility so that it may prepare itself to recover from the
expiration of a volatile filehandle. If the server returns
persistent filehandles, the client does not need these additional
steps.
For volatile filehandles, most commonly the client will need to store
the component names leading up to and including the file system
object in question. With these names, the client should be able to
recover by finding a filehandle in the namespace that is still
available or by starting at the root of the server's file system
namespace.
If the expired filehandle refers to an object that has been removed
from the file system, obviously the client will not be able to
recover from the expired filehandle.
It is also possible that the expired filehandle refers to a file that
has been renamed. If the file was renamed by another client, again
it is possible that the original client will not be able to recover.
However, in the case that the client itself is renaming the file and
the file is open, it is possible that the client may be able to
recover. The client can determine the new pathname based on the
processing of the rename request. The client can then regenerate the
new filehandle based on the new pathname. The client could also use
the COMPOUND procedure to construct a series of operations like:
RENAME A B
LOOKUP B
GETFH
Note that the COMPOUND procedure does not provide atomicity. This
example only reduces the overhead of recovering from an expired
filehandle.
5. File Attributes
To meet the requirements of extensibility and increased
interoperability with non-UNIX platforms, attributes need to be
handled in a flexible manner. The NFSv3 fattr3 structure contains a
fixed list of attributes that not all clients and servers are able to
support or care about. The fattr3 structure cannot be extended as
new needs arise and it provides no way to indicate non-support. With
the NFSv4.1 protocol, the client is able to query what attributes the
server supports and construct requests with only those supported
attributes (or a subset thereof).
To this end, attributes are divided into three groups: REQUIRED,
RECOMMENDED, and named. Both REQUIRED and RECOMMENDED attributes are
supported in the NFSv4.1 protocol by a specific and well-defined
encoding and are identified by number. They are requested by setting
a bit in the bit vector sent in the GETATTR request; the server
response includes a bit vector to list what attributes were returned
in the response. New REQUIRED or RECOMMENDED attributes may be added
to the NFSv4 protocol as part of a new minor version by publishing a
Standards Track RFC that allocates a new attribute number value and
defines the encoding for the attribute. See Section 2.7 for further
discussion.
Named attributes are accessed by the new OPENATTR operation, which
accesses a hidden directory of attributes associated with a file
system object. OPENATTR takes a filehandle for the object and
returns the filehandle for the attribute hierarchy. The filehandle
for the named attributes is a directory object accessible by LOOKUP
or READDIR and contains files whose names represent the named
attributes and whose data bytes are the value of the attribute. For
example:
+----------+-----------+---------------------------------+
| LOOKUP | "foo" | ; look up file |
| GETATTR | attrbits | |
| OPENATTR | | ; access foo's named attributes |
| LOOKUP | "x11icon" | ; look up specific attribute |
| READ | 0,4096 | ; read stream of bytes |
+----------+-----------+---------------------------------+
Named attributes are intended for data needed by applications rather than by an NFS client implementation. NFS implementors are strongly encouraged to define their new attributes as RECOMMENDED attributes by bringing them to the IETF Standards Track process. The set of attributes that are classified as REQUIRED is deliberately small since servers need to do whatever it takes to support them. A server should support as many of the RECOMMENDED attributes as possible but, by their definition, the server is not required to support all of them. Attributes are deemed REQUIRED if the data is both needed by a large number of clients and is not otherwise reasonably computable by the client when support is not provided on the server. Note that the hidden directory returned by OPENATTR is a convenience for protocol processing. The client should not make any assumptions about the server's implementation of named attributes and whether or not the underlying file system at the server has a named attribute directory. Therefore, operations such as SETATTR and GETATTR on the named attribute directory are undefined.5.1. REQUIRED Attributes
These MUST be supported by every NFSv4.1 client and server in order to ensure a minimum level of interoperability. The server MUST store and return these attributes, and the client MUST be able to function with an attribute set limited to these attributes. With just the REQUIRED attributes some client functionality may be impaired or limited in some ways. A client may ask for any of these attributes to be returned by setting a bit in the GETATTR request, and the server MUST return their value.5.2. RECOMMENDED Attributes
These attributes are understood well enough to warrant support in the NFSv4.1 protocol. However, they may not be supported on all clients and servers. A client may ask for any of these attributes to be returned by setting a bit in the GETATTR request but must handle the case where the server does not return them. A client MAY ask for the set of attributes the server supports and SHOULD NOT request attributes the server does not support. A server should be tolerant of requests for unsupported attributes and simply not return them rather than considering the request an error. It is expected that servers will support all attributes they comfortably can and only fail to support attributes that are difficult to support in their operating environments. A server should provide attributes whenever they don't have to "tell lies" to the client. For example, a file modification time should be either an accurate time or should not be
supported by the server. At times this will be difficult for clients, but a client is better positioned to decide whether and how to fabricate or construct an attribute or whether to do without the attribute.5.3. Named Attributes
These attributes are not supported by direct encoding in the NFSv4 protocol but are accessed by string names rather than numbers and correspond to an uninterpreted stream of bytes that are stored with the file system object. The namespace for these attributes may be accessed by using the OPENATTR operation. The OPENATTR operation returns a filehandle for a virtual "named attribute directory", and further perusal and modification of the namespace may be done using operations that work on more typical directories. In particular, READDIR may be used to get a list of such named attributes, and LOOKUP and OPEN may select a particular attribute. Creation of a new named attribute may be the result of an OPEN specifying file creation. Once an OPEN is done, named attributes may be examined and changed by normal READ and WRITE operations using the filehandles and stateids returned by OPEN. Named attributes and the named attribute directory may have their own (non-named) attributes. Each of these objects MUST have all of the REQUIRED attributes and may have additional RECOMMENDED attributes. However, the set of attributes for named attributes and the named attribute directory need not be, and typically will not be, as large as that for other objects in that file system. Named attributes and the named attribute directory might be the target of delegations (in the case of the named attribute directory, these will be directory delegations). However, since granting delegations is at the server's discretion, a server need not support delegations on named attributes or the named attribute directory. It is RECOMMENDED that servers support arbitrary named attributes. A client should not depend on the ability to store any named attributes in the server's file system. If a server does support named attributes, a client that is also able to handle them should be able to copy a file's data and metadata with complete transparency from one location to another; this would imply that names allowed for regular directory entries are valid for named attribute names as well.
In NFSv4.1, the structure of named attribute directories is
restricted in a number of ways, in order to prevent the development
of non-interoperable implementations in which some servers support a
fully general hierarchical directory structure for named attributes
while others support a limited but adequate structure for named
attributes. In such an environment, clients or applications might
come to depend on non-portable extensions. The restrictions are:
o CREATE is not allowed in a named attribute directory. Thus, such
objects as symbolic links and special files are not allowed to be
named attributes. Further, directories may not be created in a
named attribute directory, so no hierarchical structure of named
attributes for a single object is allowed.
o If OPENATTR is done on a named attribute directory or on a named
attribute, the server MUST return NFS4ERR_WRONG_TYPE.
o Doing a RENAME of a named attribute to a different named attribute
directory or to an ordinary (i.e., non-named-attribute) directory
is not allowed.
o Creating hard links between named attribute directories or between
named attribute directories and ordinary directories is not
allowed.
Names of attributes will not be controlled by this document or other
IETF Standards Track documents. See Section 22.1 for further
discussion.
5.4. Classification of Attributes
Each of the REQUIRED and RECOMMENDED attributes can be classified in
one of three categories: per server (i.e., the value of the attribute
will be the same for all file objects that share the same server
owner; see Section 2.5 for a definition of server owner), per file
system (i.e., the value of the attribute will be the same for some or
all file objects that share the same fsid attribute (Section 5.8.1.9)
and server owner), or per file system object. Note that it is
possible that some per file system attributes may vary within the
file system, depending on the value of the "homogeneous"
(Section 5.8.2.16) attribute. Note that the attributes
time_access_set and time_modify_set are not listed in this section
because they are write-only attributes corresponding to time_access
and time_modify, and are used in a special instance of SETATTR.
o The per-server attribute is:
lease_time
o The per-file system attributes are:
supported_attrs, suppattr_exclcreat, fh_expire_type,
link_support, symlink_support, unique_handles, aclsupport,
cansettime, case_insensitive, case_preserving,
chown_restricted, files_avail, files_free, files_total,
fs_locations, homogeneous, maxfilesize, maxname, maxread,
maxwrite, no_trunc, space_avail, space_free, space_total,
time_delta, change_policy, fs_status, fs_layout_type,
fs_locations_info, fs_charset_cap
o The per-file system object attributes are:
type, change, size, named_attr, fsid, rdattr_error, filehandle,
acl, archive, fileid, hidden, maxlink, mimetype, mode,
numlinks, owner, owner_group, rawdev, space_used, system,
time_access, time_backup, time_create, time_metadata,
time_modify, mounted_on_fileid, dir_notif_delay,
dirent_notif_delay, dacl, sacl, layout_type, layout_hint,
layout_blksize, layout_alignment, mdsthreshold, retention_get,
retention_set, retentevt_get, retentevt_set, retention_hold,
mode_set_masked
For quota_avail_hard, quota_avail_soft, and quota_used, see their
definitions below for the appropriate classification.
5.5. Set-Only and Get-Only Attributes
Some REQUIRED and RECOMMENDED attributes are set-only; i.e., they can
be set via SETATTR but not retrieved via GETATTR. Similarly, some
REQUIRED and RECOMMENDED attributes are get-only; i.e., they can be
retrieved via GETATTR but not set via SETATTR. If a client attempts
to set a get-only attribute or get a set-only attributes, the server
MUST return NFS4ERR_INVAL.
5.6. REQUIRED Attributes - List and Definition References
The list of REQUIRED attributes appears in Table 2. The meaning of
the columns of the table are:
o Name: The name of the attribute.
o Id: The number assigned to the attribute. In the event of
conflicts between the assigned number and [13], the latter is
likely authoritative, but should be resolved with Errata to this
document and/or [13]. See [44] for the Errata process.
o Data Type: The XDR data type of the attribute.
o Acc: Access allowed to the attribute. R means read-only (GETATTR
may retrieve, SETATTR may not set). W means write-only (SETATTR
may set, GETATTR may not retrieve). R W means read/write (GETATTR
may retrieve, SETATTR may set).
o Defined in: The section of this specification that describes the
attribute.
+--------------------+----+------------+-----+------------------+
| Name | Id | Data Type | Acc | Defined in: |
+--------------------+----+------------+-----+------------------+
| supported_attrs | 0 | bitmap4 | R | Section 5.8.1.1 |
| type | 1 | nfs_ftype4 | R | Section 5.8.1.2 |
| fh_expire_type | 2 | uint32_t | R | Section 5.8.1.3 |
| change | 3 | uint64_t | R | Section 5.8.1.4 |
| size | 4 | uint64_t | R W | Section 5.8.1.5 |
| link_support | 5 | bool | R | Section 5.8.1.6 |
| symlink_support | 6 | bool | R | Section 5.8.1.7 |
| named_attr | 7 | bool | R | Section 5.8.1.8 |
| fsid | 8 | fsid4 | R | Section 5.8.1.9 |
| unique_handles | 9 | bool | R | Section 5.8.1.10 |
| lease_time | 10 | nfs_lease4 | R | Section 5.8.1.11 |
| rdattr_error | 11 | enum | R | Section 5.8.1.12 |
| filehandle | 19 | nfs_fh4 | R | Section 5.8.1.13 |
| suppattr_exclcreat | 75 | bitmap4 | R | Section 5.8.1.14 |
+--------------------+----+------------+-----+------------------+
Table 2
5.7. RECOMMENDED Attributes - List and Definition References
The RECOMMENDED attributes are defined in Table 3. The meanings of
the column headers are the same as Table 2; see Section 5.6 for the
meanings.
+--------------------+----+----------------+-----+------------------+
| Name | Id | Data Type | Acc | Defined in: |
+--------------------+----+----------------+-----+------------------+
| acl | 12 | nfsace4<> | R W | Section 6.2.1 |
| aclsupport | 13 | uint32_t | R | Section 6.2.1.2 |
| archive | 14 | bool | R W | Section 5.8.2.1 |
| cansettime | 15 | bool | R | Section 5.8.2.2 |
| case_insensitive | 16 | bool | R | Section 5.8.2.3 |
| case_preserving | 17 | bool | R | Section 5.8.2.4 |
| change_policy | 60 | chg_policy4 | R | Section 5.8.2.5 |
| chown_restricted | 18 | bool | R | Section 5.8.2.6 |
| dacl | 58 | nfsacl41 | R W | Section 6.2.2 |
| dir_notif_delay | 56 | nfstime4 | R | Section 5.11.1 |
| dirent_notif_delay | 57 | nfstime4 | R | Section 5.11.2 |
| fileid | 20 | uint64_t | R | Section 5.8.2.7 |
| files_avail | 21 | uint64_t | R | Section 5.8.2.8 |
| files_free | 22 | uint64_t | R | Section 5.8.2.9 |
| files_total | 23 | uint64_t | R | Section 5.8.2.10 |
| fs_charset_cap | 76 | uint32_t | R | Section 5.8.2.11 |
| fs_layout_type | 62 | layouttype4<> | R | Section 5.12.1 |
| fs_locations | 24 | fs_locations | R | Section 5.8.2.12 |
| fs_locations_info | 67 | * | R | Section 5.8.2.13 |
| fs_status | 61 | fs4_status | R | Section 5.8.2.14 |
| hidden | 25 | bool | R W | Section 5.8.2.15 |
| homogeneous | 26 | bool | R | Section 5.8.2.16 |
| layout_alignment | 66 | uint32_t | R | Section 5.12.2 |
| layout_blksize | 65 | uint32_t | R | Section 5.12.3 |
| layout_hint | 63 | layouthint4 | W | Section 5.12.4 |
| layout_type | 64 | layouttype4<> | R | Section 5.12.5 |
| maxfilesize | 27 | uint64_t | R | Section 5.8.2.17 |
| maxlink | 28 | uint32_t | R | Section 5.8.2.18 |
| maxname | 29 | uint32_t | R | Section 5.8.2.19 |
| maxread | 30 | uint64_t | R | Section 5.8.2.20 |
| maxwrite | 31 | uint64_t | R | Section 5.8.2.21 |
| mdsthreshold | 68 | mdsthreshold4 | R | Section 5.12.6 |
| mimetype | 32 | utf8str_cs | R W | Section 5.8.2.22 |
| mode | 33 | mode4 | R W | Section 6.2.4 |
| mode_set_masked | 74 | mode_masked4 | W | Section 6.2.5 |
| mounted_on_fileid | 55 | uint64_t | R | Section 5.8.2.23 |
| no_trunc | 34 | bool | R | Section 5.8.2.24 |
| numlinks | 35 | uint32_t | R | Section 5.8.2.25 |
| owner | 36 | utf8str_mixed | R W | Section 5.8.2.26 |
| owner_group | 37 | utf8str_mixed | R W | Section 5.8.2.27 |
| quota_avail_hard | 38 | uint64_t | R | Section 5.8.2.28 |
| quota_avail_soft | 39 | uint64_t | R | Section 5.8.2.29 |
| quota_used | 40 | uint64_t | R | Section 5.8.2.30 |
| rawdev | 41 | specdata4 | R | Section 5.8.2.31 |
| retentevt_get | 71 | retention_get4 | R | Section 5.13.3 |
| retentevt_set | 72 | retention_set4 | W | Section 5.13.4 | | retention_get | 69 | retention_get4 | R | Section 5.13.1 | | retention_hold | 73 | uint64_t | R W | Section 5.13.5 | | retention_set | 70 | retention_set4 | W | Section 5.13.2 | | sacl | 59 | nfsacl41 | R W | Section 6.2.3 | | space_avail | 42 | uint64_t | R | Section 5.8.2.32 | | space_free | 43 | uint64_t | R | Section 5.8.2.33 | | space_total | 44 | uint64_t | R | Section 5.8.2.34 | | space_used | 45 | uint64_t | R | Section 5.8.2.35 | | system | 46 | bool | R W | Section 5.8.2.36 | | time_access | 47 | nfstime4 | R | Section 5.8.2.37 | | time_access_set | 48 | settime4 | W | Section 5.8.2.38 | | time_backup | 49 | nfstime4 | R W | Section 5.8.2.39 | | time_create | 50 | nfstime4 | R W | Section 5.8.2.40 | | time_delta | 51 | nfstime4 | R | Section 5.8.2.41 | | time_metadata | 52 | nfstime4 | R | Section 5.8.2.42 | | time_modify | 53 | nfstime4 | R | Section 5.8.2.43 | | time_modify_set | 54 | settime4 | W | Section 5.8.2.44 | +--------------------+----+----------------+-----+------------------+ Table 3 * fs_locations_info45.8. Attribute Definitions
5.8.1. Definitions of REQUIRED Attributes
5.8.1.1. Attribute 0: supported_attrs
The bit vector that would retrieve all REQUIRED and RECOMMENDED attributes that are supported for this object. The scope of this attribute applies to all objects with a matching fsid.5.8.1.2. Attribute 1: type
Designates the type of an object in terms of one of a number of special constants: o NF4REG designates a regular file. o NF4DIR designates a directory. o NF4BLK designates a block device special file. o NF4CHR designates a character device special file. o NF4LNK designates a symbolic link.
o NF4SOCK designates a named socket special file.
o NF4FIFO designates a fifo special file.
o NF4ATTRDIR designates a named attribute directory.
o NF4NAMEDATTR designates a named attribute.
Within the explanatory text and operation descriptions, the following
phrases will be used with the meanings given below:
o The phrase "is a directory" means that the object's type attribute
is NF4DIR or NF4ATTRDIR.
o The phrase "is a special file" means that the object's type
attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.
o The phrases "is an ordinary file" and "is a regular file" mean
that the object's type attribute is NF4REG or NF4NAMEDATTR.
5.8.1.3. Attribute 2: fh_expire_type
Server uses this to specify filehandle expiration behavior to the
client. See Section 4 for additional description.
5.8.1.4. Attribute 3: change
A value created by the server that the client can use to determine if
file data, directory contents, or attributes of the object have been
modified. The server may return the object's time_metadata attribute
for this attribute's value, but only if the file system object cannot
be updated more frequently than the resolution of time_metadata.
5.8.1.5. Attribute 4: size
The size of the object in bytes.
5.8.1.6. Attribute 5: link_support
TRUE, if the object's file system supports hard links.
5.8.1.7. Attribute 6: symlink_support
TRUE, if the object's file system supports symbolic links.
5.8.1.8. Attribute 7: named_attr
TRUE, if this object has named attributes. In other words, object has a non-empty named attribute directory.5.8.1.9. Attribute 8: fsid
Unique file system identifier for the file system holding this object. The fsid attribute has major and minor components, each of which are of data type uint64_t.5.8.1.10. Attribute 9: unique_handles
TRUE, if two distinct filehandles are guaranteed to refer to two different file system objects.5.8.1.11. Attribute 10: lease_time
Duration of the lease at server in seconds.5.8.1.12. Attribute 11: rdattr_error
Error returned from an attempt to retrieve attributes during a READDIR operation.5.8.1.13. Attribute 19: filehandle
The filehandle of this object (primarily for READDIR requests).5.8.1.14. Attribute 75: suppattr_exclcreat
The bit vector that would set all REQUIRED and RECOMMENDED attributes that are supported by the EXCLUSIVE4_1 method of file creation via the OPEN operation. The scope of this attribute applies to all objects with a matching fsid.5.8.2. Definitions of Uncategorized RECOMMENDED Attributes
The definitions of most of the RECOMMENDED attributes follow. Collections that share a common category are defined in other sections.5.8.2.1. Attribute 14: archive
TRUE, if this file has been archived since the time of last modification (deprecated in favor of time_backup).
5.8.2.2. Attribute 15: cansettime
TRUE, if the server is able to change the times for a file system object as specified in a SETATTR operation.5.8.2.3. Attribute 16: case_insensitive
TRUE, if file name comparisons on this file system are case insensitive.5.8.2.4. Attribute 17: case_preserving
TRUE, if file name case on this file system is preserved.5.8.2.5. Attribute 60: change_policy
A value created by the server that the client can use to determine if some server policy related to the current file system has been subject to change. If the value remains the same, then the client can be sure that the values of the attributes related to fs location and the fss_type field of the fs_status attribute have not changed. On the other hand, a change in this value does necessarily imply a change in policy. It is up to the client to interrogate the server to determine if some policy relevant to it has changed. See Section 3.3.6 for details. This attribute MUST change when the value returned by the fs_locations or fs_locations_info attribute changes, when a file system goes from read-only to writable or vice versa, or when the allowable set of security flavors for the file system or any part thereof is changed.5.8.2.6. Attribute 18: chown_restricted
If TRUE, the server will reject any request to change either the owner or the group associated with a file if the caller is not a privileged user (for example, "root" in UNIX operating environments or, in Windows 2000, the "Take Ownership" privilege).5.8.2.7. Attribute 20: fileid
A number uniquely identifying the file within the file system.5.8.2.8. Attribute 21: files_avail
File slots available to this user on the file system containing this object -- this should be the smallest relevant limit.
5.8.2.9. Attribute 22: files_free
Free file slots on the file system containing this object -- this should be the smallest relevant limit.5.8.2.10. Attribute 23: files_total
Total file slots on the file system containing this object.5.8.2.11. Attribute 76: fs_charset_cap
Character set capabilities for this file system. See Section 14.4.5.8.2.12. Attribute 24: fs_locations
Locations where this file system may be found. If the server returns NFS4ERR_MOVED as an error, this attribute MUST be supported. See Section 11.9 for more details.5.8.2.13. Attribute 67: fs_locations_info
Full function file system location. See Section 11.10 for more details.5.8.2.14. Attribute 61: fs_status
Generic file system type information. See Section 11.11 for more details.5.8.2.15. Attribute 25: hidden
TRUE, if the file is considered hidden with respect to the Windows API.5.8.2.16. Attribute 26: homogeneous
TRUE, if this object's file system is homogeneous; i.e., all objects in the file system (all objects on the server with the same fsid) have common values for all per-file-system attributes.5.8.2.17. Attribute 27: maxfilesize
Maximum supported file size for the file system of this object.5.8.2.18. Attribute 28: maxlink
Maximum number of links for this object.
5.8.2.19. Attribute 29: maxname
Maximum file name size supported for this object.5.8.2.20. Attribute 30: maxread
Maximum amount of data the READ operation will return for this object.5.8.2.21. Attribute 31: maxwrite
Maximum amount of data the WRITE operation will accept for this object. This attribute SHOULD be supported if the file is writable. Lack of this attribute can lead to the client either wasting bandwidth or not receiving the best performance.5.8.2.22. Attribute 32: mimetype
MIME body type/subtype of this object.5.8.2.23. Attribute 55: mounted_on_fileid
Like fileid, but if the target filehandle is the root of a file system, this attribute represents the fileid of the underlying directory. UNIX-based operating environments connect a file system into the namespace by connecting (mounting) the file system onto the existing file object (the mount point, usually a directory) of an existing file system. When the mount point's parent directory is read via an API like readdir(), the return results are directory entries, each with a component name and a fileid. The fileid of the mount point's directory entry will be different from the fileid that the stat() system call returns. The stat() system call is returning the fileid of the root of the mounted file system, whereas readdir() is returning the fileid that stat() would have returned before any file systems were mounted on the mount point. Unlike NFSv3, NFSv4.1 allows a client's LOOKUP request to cross other file systems. The client detects the file system crossing whenever the filehandle argument of LOOKUP has an fsid attribute different from that of the filehandle returned by LOOKUP. A UNIX-based client will consider this a "mount point crossing". UNIX has a legacy scheme for allowing a process to determine its current working directory. This relies on readdir() of a mount point's parent and stat() of the mount point returning fileids as previously described. The mounted_on_fileid attribute corresponds to the fileid that readdir() would have returned as described previously.
While the NFSv4.1 client could simply fabricate a fileid corresponding to what mounted_on_fileid provides (and if the server does not support mounted_on_fileid, the client has no choice), there is a risk that the client will generate a fileid that conflicts with one that is already assigned to another object in the file system. Instead, if the server can provide the mounted_on_fileid, the potential for client operational problems in this area is eliminated. If the server detects that there is no mounted point at the target file object, then the value for mounted_on_fileid that it returns is the same as that of the fileid attribute. The mounted_on_fileid attribute is RECOMMENDED, so the server SHOULD provide it if possible, and for a UNIX-based server, this is straightforward. Usually, mounted_on_fileid will be requested during a READDIR operation, in which case it is trivial (at least for UNIX- based servers) to return mounted_on_fileid since it is equal to the fileid of a directory entry returned by readdir(). If mounted_on_fileid is requested in a GETATTR operation, the server should obey an invariant that has it returning a value that is equal to the file object's entry in the object's parent directory, i.e., what readdir() would have returned. Some operating environments allow a series of two or more file systems to be mounted onto a single mount point. In this case, for the server to obey the aforementioned invariant, it will need to find the base mount point, and not the intermediate mount points.5.8.2.24. Attribute 34: no_trunc
If this attribute is TRUE, then if the client uses a file name longer than name_max, an error will be returned instead of the name being truncated.5.8.2.25. Attribute 35: numlinks
Number of hard links to this object.5.8.2.26. Attribute 36: owner
The string name of the owner of this object.5.8.2.27. Attribute 37: owner_group
The string name of the group ownership of this object.
5.8.2.28. Attribute 38: quota_avail_hard
The value in bytes that represents the amount of additional disk space beyond the current allocation that can be allocated to this file or directory before further allocations will be refused. It is understood that this space may be consumed by allocations to other files or directories.5.8.2.29. Attribute 39: quota_avail_soft
The value in bytes that represents the amount of additional disk space that can be allocated to this file or directory before the user may reasonably be warned. It is understood that this space may be consumed by allocations to other files or directories though there is a rule as to which other files or directories.5.8.2.30. Attribute 40: quota_used
The value in bytes that represents the amount of disk space used by this file or directory and possibly a number of other similar files or directories, where the set of "similar" meets at least the criterion that allocating space to any file or directory in the set will reduce the "quota_avail_hard" of every other file or directory in the set. Note that there may be a number of distinct but overlapping sets of files or directories for which a quota_used value is maintained, e.g., "all files with a given owner", "all files with a given group owner", etc. The server is at liberty to choose any of those sets when providing the content of the quota_used attribute, but should do so in a repeatable way. The rule may be configured per file system or may be "choose the set with the smallest quota".5.8.2.31. Attribute 41: rawdev
Raw device number of file of type NF4BLK or NF4CHR. The device number is split into major and minor numbers. If the file's type attribute is not NF4BLK or NF4CHR, the value returned SHOULD NOT be considered useful.5.8.2.32. Attribute 42: space_avail
Disk space in bytes available to this user on the file system containing this object -- this should be the smallest relevant limit.
5.8.2.33. Attribute 43: space_free
Free disk space in bytes on the file system containing this object -- this should be the smallest relevant limit.5.8.2.34. Attribute 44: space_total
Total disk space in bytes on the file system containing this object.5.8.2.35. Attribute 45: space_used
Number of file system bytes allocated to this object.5.8.2.36. Attribute 46: system
This attribute is TRUE if this file is a "system" file with respect to the Windows operating environment.5.8.2.37. Attribute 47: time_access
The time_access attribute represents the time of last access to the object by a READ operation sent to the server. The notion of what is an "access" depends on the server's operating environment and/or the server's file system semantics. For example, for servers obeying Portable Operating System Interface (POSIX) semantics, time_access would be updated only by the READ and READDIR operations and not any of the operations that modify the content of the object [16], [17], [18]. Of course, setting the corresponding time_access_set attribute is another way to modify the time_access attribute. Whenever the file object resides on a writable file system, the server should make its best efforts to record time_access into stable storage. However, to mitigate the performance effects of doing so, and most especially whenever the server is satisfying the read of the object's content from its cache, the server MAY cache access time updates and lazily write them to stable storage. It is also acceptable to give administrators of the server the option to disable time_access updates.5.8.2.38. Attribute 48: time_access_set
Sets the time of last access to the object. SETATTR use only.5.8.2.39. Attribute 49: time_backup
The time of last backup of the object.
5.8.2.40. Attribute 50: time_create
The time of creation of the object. This attribute does not have any relation to the traditional UNIX file attribute "ctime" or "change time".5.8.2.41. Attribute 51: time_delta
Smallest useful server time granularity.5.8.2.42. Attribute 52: time_metadata
The time of last metadata modification of the object.5.8.2.43. Attribute 53: time_modify
The time of last modification to the object.5.8.2.44. Attribute 54: time_modify_set
Sets the time of last modification to the object. SETATTR use only.5.9. Interpreting owner and owner_group
The RECOMMENDED attributes "owner" and "owner_group" (and also users and groups within the "acl" attribute) are represented in terms of a UTF-8 string. To avoid a representation that is tied to a particular underlying implementation at the client or server, the use of the UTF-8 string has been chosen. Note that Section 6.1 of RFC 2624 [45] provides additional rationale. It is expected that the client and server will have their own local representation of owner and owner_group that is used for local storage or presentation to the end user. Therefore, it is expected that when these attributes are transferred between the client and server, the local representation is translated to a syntax of the form "user@dns_domain". This will allow for a client and server that do not use the same local representation the ability to translate to a common syntax that can be interpreted by both. Similarly, security principals may be represented in different ways by different security mechanisms. Servers normally translate these representations into a common format, generally that used by local storage, to serve as a means of identifying the users corresponding to these security principals. When these local identifiers are translated to the form of the owner attribute, associated with files created by such principals, they identify, in a common format, the users associated with each corresponding set of security principals.
The translation used to interpret owner and group strings is not specified as part of the protocol. This allows various solutions to be employed. For example, a local translation table may be consulted that maps a numeric identifier to the user@dns_domain syntax. A name service may also be used to accomplish the translation. A server may provide a more general service, not limited by any particular translation (which would only translate a limited set of possible strings) by storing the owner and owner_group attributes in local storage without any translation or it may augment a translation method by storing the entire string for attributes for which no translation is available while using the local representation for those cases in which a translation is available. Servers that do not provide support for all possible values of the owner and owner_group attributes SHOULD return an error (NFS4ERR_BADOWNER) when a string is presented that has no translation, as the value to be set for a SETATTR of the owner, owner_group, or acl attributes. When a server does accept an owner or owner_group value as valid on a SETATTR (and similarly for the owner and group strings in an acl), it is promising to return that same string when a corresponding GETATTR is done. Configuration changes (including changes from the mapping of the string to the local representation) and ill-constructed name translations (those that contain aliasing) may make that promise impossible to honor. Servers should make appropriate efforts to avoid a situation in which these attributes have their values changed when no real change to ownership has occurred. The "dns_domain" portion of the owner string is meant to be a DNS domain name, for example, user@example.org. Servers should accept as valid a set of users for at least one domain. A server may treat other domains as having no valid translations. A more general service is provided when a server is capable of accepting users for multiple domains, or for all domains, subject to security constraints. In the case where there is no translation available to the client or server, the attribute value will be constructed without the "@". Therefore, the absence of the @ from the owner or owner_group attribute signifies that no translation was available at the sender and that the receiver of the attribute should not use that string as a basis for translation into its own internal format. Even though the attribute value cannot be translated, it may still be useful. In the case of a client, the attribute string may be used for local display of ownership.
To provide a greater degree of compatibility with NFSv3, which identified users and groups by 32-bit unsigned user identifiers and group identifiers, owner and group strings that consist of decimal numeric values with no leading zeros can be given a special interpretation by clients and servers that choose to provide such support. The receiver may treat such a user or group string as representing the same user as would be represented by an NFSv3 uid or gid having the corresponding numeric value. A server is not obligated to accept such a string, but may return an NFS4ERR_BADOWNER instead. To avoid this mechanism being used to subvert user and group translation, so that a client might pass all of the owners and groups in numeric form, a server SHOULD return an NFS4ERR_BADOWNER error when there is a valid translation for the user or owner designated in this way. In that case, the client must use the appropriate name@domain string and not the special form for compatibility. The owner string "nobody" may be used to designate an anonymous user, which will be associated with a file created by a security principal that cannot be mapped through normal means to the owner attribute. Users and implementations of NFSv4.1 SHOULD NOT use "nobody" to designate a real user whose access is not anonymous.5.10. Character Case Attributes
With respect to the case_insensitive and case_preserving attributes, each UCS-4 character (which UTF-8 encodes) can be mapped according to Appendix B.2 of RFC 3454 [19]. For general character handling and internationalization issues, see Section 14.5.11. Directory Notification Attributes
As described in Section 18.39, the client can request a minimum delay for notifications of changes to attributes, but the server is free to ignore what the client requests. The client can determine in advance what notification delays the server will accept by sending a GETATTR operation for either or both of two directory notification attributes. When the client calls the GET_DIR_DELEGATION operation and asks for attribute change notifications, it should request notification delays that are no less than the values in the server- provided attributes.5.11.1. Attribute 56: dir_notif_delay
The dir_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to the directory's attributes.
5.11.2. Attribute 57: dirent_notif_delay
The dirent_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to a file object that has an entry in the directory.5.12. pNFS Attribute Definitions
5.12.1. Attribute 62: fs_layout_type
The fs_layout_type attribute (see Section 3.3.13) applies to a file system and indicates what layout types are supported by the file system. When the client encounters a new fsid, the client SHOULD obtain the value for the fs_layout_type attribute associated with the new file system. This attribute is used by the client to determine if the layout types supported by the server match any of the client's supported layout types.5.12.2. Attribute 66: layout_alignment
When a client holds layouts on files of a file system, the layout_alignment attribute indicates the preferred alignment for I/O to files on that file system. Where possible, the client should send READ and WRITE operations with offsets that are whole multiples of the layout_alignment attribute.5.12.3. Attribute 65: layout_blksize
When a client holds layouts on files of a file system, the layout_blksize attribute indicates the preferred block size for I/O to files on that file system. Where possible, the client should send READ operations with a count argument that is a whole multiple of layout_blksize, and WRITE operations with a data argument of size that is a whole multiple of layout_blksize.5.12.4. Attribute 63: layout_hint
The layout_hint attribute (see Section 3.3.19) may be set on newly created files to influence the metadata server's choice for the file's layout. If possible, this attribute is one of those set in the initial attributes within the OPEN operation. The metadata server may choose to ignore this attribute. The layout_hint attribute is a subset of the layout structure returned by LAYOUTGET. For example, instead of specifying particular devices, this would be used to suggest the stripe width of a file. The server implementation determines which fields within the layout will be used.
5.12.5. Attribute 64: layout_type
This attribute lists the layout type(s) available for a file. The value returned by the server is for informational purposes only. The client will use the LAYOUTGET operation to obtain the information needed in order to perform I/O, for example, the specific device information for the file and its layout.5.12.6. Attribute 68: mdsthreshold
This attribute is a server-provided hint used to communicate to the client when it is more efficient to send READ and WRITE operations to the metadata server or the data server. The two types of thresholds described are file size thresholds and I/O size thresholds. If a file's size is smaller than the file size threshold, data accesses SHOULD be sent to the metadata server. If an I/O request has a length that is below the I/O size threshold, the I/O SHOULD be sent to the metadata server. Each threshold type is specified separately for read and write. The server MAY provide both types of thresholds for a file. If both file size and I/O size are provided, the client SHOULD reach or exceed both thresholds before sending its read or write requests to the data server. Alternatively, if only one of the specified thresholds is reached or exceeded, the I/O requests are sent to the metadata server. For each threshold type, a value of zero indicates no READ or WRITE should be sent to the metadata server, while a value of all ones indicates that all READs or WRITEs should be sent to the metadata server. The attribute is available on a per-filehandle basis. If the current filehandle refers to a non-pNFS file or directory, the metadata server should return an attribute that is representative of the filehandle's file system. It is suggested that this attribute is queried as part of the OPEN operation. Due to dynamic system changes, the client should not assume that the attribute will remain constant for any specific time period; thus, it should be periodically refreshed.5.13. Retention Attributes
Retention is a concept whereby a file object can be placed in an immutable, undeletable, unrenamable state for a fixed or infinite duration of time. Once in this "retained" state, the file cannot be moved out of the state until the duration of retention has been reached.
When retention is enabled, retention MUST extend to the data of the file, and the name of file. The server MAY extend retention to any other property of the file, including any subset of REQUIRED, RECOMMENDED, and named attributes, with the exceptions noted in this section. Servers MAY support or not support retention on any file object type. The five retention attributes are explained in the next subsections.5.13.1. Attribute 69: retention_get
If retention is enabled for the associated file, this attribute's value represents the retention begin time of the file object. This attribute's value is only readable with the GETATTR operation and MUST NOT be modified by the SETATTR operation (Section 5.5). The value of the attribute consists of: const RET4_DURATION_INFINITE = 0xffffffffffffffff; struct retention_get4 { uint64_t rg_duration; nfstime4 rg_begin_time<1>; }; The field rg_duration is the duration in seconds indicating how long the file will be retained once retention is enabled. The field rg_begin_time is an array of up to one absolute time value. If the array is zero length, no beginning retention time has been established, and retention is not enabled. If rg_duration is equal to RET4_DURATION_INFINITE, the file, once retention is enabled, will be retained for an infinite duration. If (as soon as) rg_duration is zero, then rg_begin_time will be of zero length, and again, retention is not (no longer) enabled.5.13.2. Attribute 70: retention_set
This attribute is used to set the retention duration and optionally enable retention for the associated file object. This attribute is only modifiable via the SETATTR operation and MUST NOT be retrieved by the GETATTR operation (Section 5.5). This attribute corresponds to retention_get. The value of the attribute consists of: struct retention_set4 { bool rs_enable; uint64_t rs_duration<1>; };
If the client sets rs_enable to TRUE, then it is enabling retention
on the file object with the begin time of retention starting from the
server's current time and date. The duration of the retention can
also be provided if the rs_duration array is of length one. The
duration is the time in seconds from the begin time of retention, and
if set to RET4_DURATION_INFINITE, the file is to be retained forever.
If retention is enabled, with no duration specified in either this
SETATTR or a previous SETATTR, the duration defaults to zero seconds.
The server MAY restrict the enabling of retention or the duration of
retention on the basis of the ACE4_WRITE_RETENTION ACL permission.
The enabling of retention MUST NOT prevent the enabling of event-
based retention or the modification of the retention_hold attribute.
The following rules apply to both the retention_set and retentevt_set
attributes.
o As long as retention is not enabled, the client is permitted to
decrease the duration.
o The duration can always be set to an equal or higher value, even
if retention is enabled. Note that once retention is enabled, the
actual duration (as returned by the retention_get or retentevt_get
attributes; see Section 5.13.1 or Section 5.13.3) is constantly
counting down to zero (one unit per second), unless the duration
was set to RET4_DURATION_INFINITE. Thus, it will not be possible
for the client to precisely extend the duration on a file that has
retention enabled.
o While retention is enabled, attempts to disable retention or
decrease the retention's duration MUST fail with the error
NFS4ERR_INVAL.
o If the principal attempting to change retention_set or
retentevt_set does not have ACE4_WRITE_RETENTION permissions, the
attempt MUST fail with NFS4ERR_ACCESS.
5.13.3. Attribute 71: retentevt_get
Gets the event-based retention duration, and if enabled, the event-
based retention begin time of the file object. This attribute is
like retention_get, but refers to event-based retention. The event
that triggers event-based retention is not defined by the NFSv4.1
specification.
5.13.4. Attribute 72: retentevt_set
Sets the event-based retention duration, and optionally enables event-based retention on the file object. This attribute corresponds to retentevt_get and is like retention_set, but refers to event-based retention. When event-based retention is set, the file MUST be retained even if non-event-based retention has been set, and the duration of non-event-based retention has been reached. Conversely, when non-event-based retention has been set, the file MUST be retained even if event-based retention has been set, and the duration of event-based retention has been reached. The server MAY restrict the enabling of event-based retention or the duration of event-based retention on the basis of the ACE4_WRITE_RETENTION ACL permission. The enabling of event-based retention MUST NOT prevent the enabling of non-event-based retention or the modification of the retention_hold attribute.5.13.5. Attribute 73: retention_hold
Gets or sets administrative retention holds, one hold per bit position. This attribute allows one to 64 administrative holds, one hold per bit on the attribute. If retention_hold is not zero, then the file MUST NOT be deleted, renamed, or modified, even if the duration on enabled event or non-event-based retention has been reached. The server MAY restrict the modification of retention_hold on the basis of the ACE4_WRITE_RETENTION_HOLD ACL permission. The enabling of administration retention holds does not prevent the enabling of event-based or non-event-based retention. If the principal attempting to change retention_hold does not have ACE4_WRITE_RETENTION_HOLD permissions, the attempt MUST fail with NFS4ERR_ACCESS.
(page 126 continued on part 6)
