[LU-11962] File LSOM updates to store proper size via FLR for regular stat() usage

• Type: Improvement
• Resolution: Unresolved
• Priority: Major
• Labels: hard patch performance som
• Assignee: WC Triage
• Reporter: Patrick Farrell (Inactive)

Description

The layout manipulations required to bring an FLR file in to sync (READONLY in FLR parlance) also give SOM while the file is in sync.  This is true SOM, with no caveats, able to be used for any purpose (as distinct from lazy SOM which can only be used by tools which are aware of it).

In essence, there is no reason the SOM portion has to be associated with a replica.  Exactly the same functionality can be used just for SOM.

Because the layout state transitions for FLR require synchronous writes to the MDS each time, and because a write to the file destroys the SOM state, this is too expensive to try to use all the time. Instead, the proposal is to set it on all files a certain amount of time after they have been modified (e.g. 24h).  If there are no writes to a file for a time, and the client is returning identical size+blocks in the LSOM state at close time, we take it through the layout transitions to mark it LCM_FL_RDONLY (does not make it not writeable, just indicates the attributes are not being modified), and then it has SOM.

This would be a fairly low effort way to allow all files except those being actively modified to have true SOM and improve performance for normal stat() and similar calls.

Issue Links

• is duplicated by: LU-13248 sanity test 807 fails with '/mnt/lustre/d807.sanity/single_dd expected blocks: 1, got: 0'
• is duplicated by: LU-14461 Convert LSOM with loose size consistency into a strong consistent version
• is duplicated by: LU-18529 FLSOM: Strict SOM for closed files
• is related to: LU-12649 Tracker for ongoing FLR improvements
• is related to: LU-12337 SoM in DoM needs to work transparently
• is related to: LU-11994 Add support for LSOM in LFSCK
• is related to: LU-11554 Make stat() work with LSOM