[Lustre-devel] some thoughts on COS

[Alex Zhuravlev]

Hi,

I think it could be great if we can use LDLM for COS. at the very first view
it looks possible:
1) each server lock is tagged with client unique id (uuid/export addr/etc)
2) mds registers own blocking AST function
3) locks to be used rep-ack's aren't released upon ACK, only upon commit
4) whenever conflict is observed by LDLM at enqueue time, MDS's blocking AST
    function is called and depending on whetner conflicting locks are taken on
    behalf same or different clients, the function issues sync causing commit
    and old lock to be released later

but one use case isn't that obvious. it's OK when first lock L1 was from client
C1 with PW mode and new lock is also from C1/PW. but then we have a situation
with same client, but locks are PW then PR:
1) we wouldn't want to sync just because client does mkdir a; touch a
2) thus we have to grant PR lock (so, first problem - sometimes PW and PR doesn't
    conflict?)
3) if we cancel PW to grant PR, then we'd have to make this PR conflicting with
    any PR coming from different client?
4) changing PR to PW in order to inherit state? (client side doesn't expect such
    locks_

all of this doesn't sound like a good solution, IMHO. at least it'd require serious
changes in LDLM while we're talking about 1.6/1.8 ... so we need another way.

probably we could re-use VBR as each inode change goes with new persistent version
and version is numerically equal to transno, comparing inode's version with last
committed transno we can learn whether the inode is committed?

next problem is to learn source of change, i.e. client. in the worst case all changes
are from different clients, thus every change means sync. but if we *cache* source
information we probably can avoid majority of syncs. IOW, we don't need to track
source all the time, it should be enough if we have this information most of time.
so, storing it in in-core inode is good enough probably. following this way we don't
need to care about inode's lifetime.

thanks, Alex