On 1 Feb 2022, at 20:41, Simon Kirby wrote: > Hello! > > I noticed high traffic in an NFS environment and tracked it down to some > users who moved SQLite databases over from previously-local storage. > > The usage pattern of SQLite here seems particularly bad on NFSv3 clients, > where a combination of F_RDLCK to F_WRLCK upgrading and locking polling > is entirely discarding the cache for other processes on the same client. > > Our load balancing configuration typically sticks most file accesses to > individual hosts (NFS clients), so I figured it was time to re-evaluate > the status of NFSv4 and file delegations here, since the files could be > delegated to one client, and then maybe the page cache could work as it > does on a local file system. It turns out this isn't happening... > > First, it seems that SQLite always opens the file O_RDWR. knfsd does not > seem to create a delegation in this case; I see it only for O_RDONLY. > > Second, it seems that do_setlk() in fs/nfs/file.c always nfs_zap_caches() > unless there's a ->have_delegation(inode, FMODE_READ). That condition has > changed slightly over the years, but the basic concept of invalidating > the cache in do_setlk has been around since pre-git. > > Since it seems like there's the intention to preserve cache with a read > delegation, I wrote a simplified testcase to simulate SQLite locking. > > With the open changed to O_RDONY (and F_RDLCK only), the v3 mount and > server show "POSIX ADVISORY READ" in /proc/locks. The v4 mount shows > "DELEG ACTIVE READ" on the server and "POSIX ADVISORY READ" on the > client. > > With O_RDONLY, I can see that cache is zapped following F_RDLCK on v3 and > not zapped on v4, so this appears to be working as expected. > > With O_RDWR restored, both server and client show "POSIX ADVISORY READ" > with v3 or v4 mounts, and since there is no read delegation, the cache > gets zapped. > > RFC 8881 10.4.2 seems to talk about locking when an OPEN_DELEGATE_WRITE > delegation is present, so it seems this was perhaps intended to work. > > How far off would we be from write delegations happening here? The linux server doesn't have write delegations implemented yet, I suspect that's why you're not seeing them. Ben
