.Lustre FID
[[struct-lu-fid]]
****
[source,c]
--------
struct lu_fid {
	__u64 f_seq;
	__u32 f_oid;
	__u32 f_ver;
};
--------

A file identifier ('FID') is a 128-bit number that uniquely identifies a
resource on an MDT or OST. The FID for the resource on an MDT
identifies the file as a whole. The FID for a resource on an OST
identifies the fragment of a file assigned to that OST.

The 'f_seq' field holds the sequence number, or SEQ (see below), 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) 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.
****

.FID SEQ
[source,c]
----
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,
	FID_SEQ_START		= 0x200000000ULL,
	FID_SEQ_LOCAL_FILE	= 0x200000001ULL,
	FID_SEQ_DOT_LUSTRE	= 0x200000002ULL,
	FID_SEQ_LOCAL_NAME	= 0x200000003ULL,
	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,
	FID_SEQ_UPDATE_LOG	= 0x200000009ULL,
	FID_SEQ_UPDATE_LOG_DIR	= 0x20000000aULL,
	FID_SEQ_NORMAL		= 0x200000400ULL,
	FID_SEQ_LOV_DEFAULT	= 0xffffffffffffffffULL
};
----

****
There are several reserved ranges of FID sequence values
(summarized in the list above), to allow for interoperability with
older Lustre file systems, 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 file systems.  Since all such OST objects used an
'f_seq' value of zero these FIDs are not unique across the file system,
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
generation in FID' (IGIF) inodes allocated by MDSs before Lustre 2.0.
This corresponds to the 4 billion maximum inode number that could be
allocated for such file systems.  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 file-system-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 file system.

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.
****