[LU-14692] deprecate use of OST FID SEQ 0 for MDT0000

Blocker — Mon, 8 Jan 2024 03:31:53 +0000

Since Lustre 2.4.0 and DNE1, it has been possible to create OST objects using a different FID SEQ range for each MDT, to avoid contention during MDT object precreation.

Objects that are created by MDT0000 are put into FID SEQ 0 (O/0/d*) on all OSTs and have a filename that is the decimal FID OID in ASCII. However, SEQ=0 objects are remapped to IDIF FID SEQ (0x100000000 | (ost_idx << 16)) so that they are unique across all OSTs.

Objects that are created by other MDTs (or MDT0000 after 2^48 objects are created in SEQ 0) use a unique SEQ in the FID_SEQ_NORMAL range (> 0x200000400), and use a filename that is the hexadecimal FID OID in ASCII.

For compatibility with pre-DNE MDTs and OSTs, the use of SEQ=0 by MDT0000 was kept until now, but there has not been a reason to keep this compatibility for new filesystems. It would be better to have MDT0000 assigned a "regular" FID SEQ range at startup, so that the SEQ=0 compatibility can eventually be removed. That would ensure OST objects have "proper and unique" FIDs, and avoid the complexity of mapping between the old SEQ=0 48-bit OID values and the IDIF FIDs.

Older filesystems using SEQ=0 would eventually delete old objects in this range and/or could be forced to migrate to using new objects to clean up the remaining usage, if necessary.

Whamcloud Community JIRA

[LU-14692] deprecate use of OST FID SEQ 0 for MDT0000