Lustre File Identifiers
~~~~~~~~~~~~~~~~~~~~~~~
-[[fids]]
+[[struct-lu-fid]]
-----
+Each resource stored on a target is assigned an identifier that is
+unique to that resource.
+
+[source,c]
+--------
struct lu_fid {
__u64 f_seq;
__u32 f_oid;
__u32 f_ver;
};
+--------
+
+A file identifier ('FID') is a 128-bit numbers that uniquely identifies a
+single file or directory on an MDTs or OSTs within a single Lustre file
+system. The FID for a Lustre file or directory is the FID from the
+corresponding MDT entry for the file. Each of the data resource for that
+file will also have a FID for each corresponding OST resource on which the
+file stores data.
+
+The 'f_seq' field holds the sequence number, or SEQ, and is used in
+conjunction with the 'FID location database' (FLDB) to
+determine on which target the resource is located. All resources with the
+same 'f_seq' value will be on the same target. A target can have more
+than one 'f_seq' value assigned to it.
+
+The 'f_oid' field holds the unique 'object identifier' (OID) within the
+sequence to identify a particular object.
+
+The 'f_ver' value identifies which version of a resource is being
+identified, in the event that the resource is being updated, and
+different hosts might be referring to different versions of the same
+resource. It has never been used as of Lustre 2.8.
+
+----
+enum fid_seq {
+ FID_SEQ_OST_MDT0 = 0,
+ FID_SEQ_LLOG = 1, /* unnamed llogs */
+ FID_SEQ_ECHO = 2,
+ FID_SEQ_UNUSED_START = 3,
+ FID_SEQ_UNUSED_END = 9,
+ FID_SEQ_LLOG_NAME = 10, /* named llogs */
+ FID_SEQ_RSVD = 11,
+ FID_SEQ_IGIF = 12,
+ FID_SEQ_IGIF_MAX = 0x0ffffffffULL,
+ FID_SEQ_IDIF = 0x100000000ULL,
+ FID_SEQ_IDIF_MAX = 0x1ffffffffULL,
+ /* Normal FID sequence starts from this value, i.e. 1<<33 */
+ FID_SEQ_START = 0x200000000ULL,
+ /* sequence for local pre-defined FIDs listed in local_oid */
+ FID_SEQ_LOCAL_FILE = 0x200000001ULL,
+ FID_SEQ_DOT_LUSTRE = 0x200000002ULL,
+ /* sequence is used for local named objects FIDs generated
+ * by local_object_storage library */
+ FID_SEQ_LOCAL_NAME = 0x200000003ULL,
+ /* Because current FLD will only cache the fid sequence, instead
+ * of oid on the client side, if the FID needs to be exposed to
+ * clients sides, it needs to make sure all of fids under one
+ * sequence will be located in one MDT. */
+ FID_SEQ_SPECIAL = 0x200000004ULL,
+ FID_SEQ_QUOTA = 0x200000005ULL,
+ FID_SEQ_QUOTA_GLB = 0x200000006ULL,
+ FID_SEQ_ROOT = 0x200000007ULL, /* Located on MDT0 */
+ FID_SEQ_LAYOUT_RBTREE = 0x200000008ULL,
+ /* sequence is used for update logs of cross-MDT operation */
+ FID_SEQ_UPDATE_LOG = 0x200000009ULL,
+ /* Sequence is used for the directory under which update logs
+ * are created. */
+ FID_SEQ_UPDATE_LOG_DIR = 0x20000000aULL,
+ FID_SEQ_NORMAL = 0x200000400ULL,
+ FID_SEQ_LOV_DEFAULT = 0xffffffffffffffffULL
+};
----
-File IDs ('fids') are 128-bit numbers that uniquely identify files and
-directories on the MDTs and OSTs of a Lustre file system. The fid for
-a Lustre file or directory is the fid from the corresponding MDT entry
-for the file. Each of the data objects for that file will also have a
-fid for each corresponding piece of the file on each of the
-OSTs. Encoded in the fid is the target on which that file metadata or
-file fragment resides. The map from fid to target is in the File
-Location DataBase (FLDB).
+There are several reserved ranges of FID sequence values, to allow for
+interoperability with older Lustre filesystems, to identify "well known"
+objects for internal or external use, as well as for future expansion.
+
+The 'FID_SEQ_OST_MDT0' (0x0) range is reserved for OST objects created
+by MDT0 in non-DNE filesystems. Since all such OST objects used an
+'f_seq' value of zero these FIDs are not unique across the filesystem,
+but the reservation of 'FID_SEQ_OST_MDT0' allows these FIDs to co-exist
+with other FIDs in the same 128-bit identifier space.
+
+The 'FID_SEQ_LLOG' (0x1) range is reserved for unnamed Lustre log (llog)
+files, used only internally on the MDS since Lustre 2.4, but previously
+exposed over the network.
+
+The 'FID_SEQ_ECHO' (0x2) range is used for temporary objects for
+testing purposes such as obdfiler-survey.
+
+The 'FID_SEQ_LLOG_NAME' (0x10) range is used for named llog files such
+as configuration logs and the ChangeLog.
+
+The 'FID_SEQ_IGIF' (0xb-0xffffffff) range is reserved for 'inode and
+generation in FID' (IGIF) inodes allocated by MDSes before Lustre 2.0.
+This corresponds to the 4 billion maximum inode number that could be
+allocated for such filesystems. The 'f_oid' field for IGIF FIDs
+contains the inode version number, and as such there is normally only
+a single object for each 'f_seq' value.
+
+The 'FID_SEQ_IDIF' (0x100000000-0x1fffffffff) range is reserved for
+mapping OST objects that were created by MDT0 using 'FID_SEQ_OST_MDT0'
+to filesystem-unique FIDs. The second 16-bit field (bits 16-31) of the
+'f_seq' field contains the OST index (0-65535). The low 16-bit field
+(bits 0-15) of 'f_seq' contains the high (bits 32-47) bits of the OST
+object ID, and the 32-bit 'f_oid' field contains the low 32 bits of
+the OST object ID.
+
+The 'FID_SEQ_LOCAL_FILE' (0x200000001) range is reserved for "well
+known" objects internal to the server and is not exposed to the network.
+
+The 'FID_SEQ_DOT_LUSTRE' (0x200000002) range is reserved for files
+under the hidden ".lustre" directory in the root of the filesystem.
+
+The 'FID_SEQ_LOCAL_NAME' (0x200000003) range is reserved for objects
+internal to the server that are allocated by name.
+The 'FID_SEQ_NORMAL' (0x200000400+) range is used for normal object
+identifiers. These objects are visible in the namespace if allocated
+by an MDT, or may be OST objects.
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