On Thu, 2020-10-01 at 12:38 +0000, Trond Myklebust wrote: > On Thu, 2020-10-01 at 06:36 -0400, Jeff Layton wrote: > > On Thu, 2020-10-01 at 01:09 +0100, Daire Byrne wrote: > > > ----- On 30 Sep, 2020, at 20:30, Jeff Layton jlayton@xxxxxxxxxx > > > wrote: > > > > > > > On Tue, 2020-09-22 at 13:31 +0100, Daire Byrne wrote: > > > > > Hi, > > > > > > > > > > I just thought I'd flesh out the other two issues I have found > > > > > with re-exporting > > > > > that are ultimately responsible for the biggest performance > > > > > bottlenecks. And > > > > > both of them revolve around the caching of metadata file > > > > > lookups in the NFS > > > > > client. > > > > > > > > > > Especially for the case where we are re-exporting a server many > > > > > milliseconds > > > > > away (i.e. on-premise -> cloud), we want to be able to control > > > > > how much the > > > > > client caches metadata and file data so that it's many LAN > > > > > clients all benefit > > > > > from the re-export server only having to do the WAN lookups > > > > > once (within a > > > > > specified coherency time). > > > > > > > > > > Keeping the file data in the vfs page cache or on disk using > > > > > fscache/cachefiles > > > > > is fairly straightforward, but keeping the metadata cached is > > > > > particularly > > > > > difficult. And without the cached metadata we introduce long > > > > > delays before we > > > > > can serve the already present and locally cached file data to > > > > > many waiting > > > > > clients. > > > > > > > > > > ----- On 7 Sep, 2020, at 18:31, Daire Byrne daire@xxxxxxxx > > > > > wrote: > > > > > > 2) If we cache metadata on the re-export server using > > > > > > actimeo=3600,nocto we can > > > > > > cut the network packets back to the origin server to zero for > > > > > > repeated lookups. > > > > > > However, if a client of the re-export server walks paths and > > > > > > memory maps those > > > > > > files (i.e. loading an application), the re-export server > > > > > > starts issuing > > > > > > unexpected calls back to the origin server again, > > > > > > ignoring/invalidating the > > > > > > re-export server's NFS client cache. We worked around this > > > > > > this by patching an > > > > > > inode/iversion validity check in inode.c so that the NFS > > > > > > client cache on the > > > > > > re-export server is used. I'm not sure about the correctness > > > > > > of this patch but > > > > > > it works for our corner case. > > > > > > > > > > If we use actimeo=3600,nocto (say) to mount a remote software > > > > > volume on the > > > > > re-export server, we can successfully cache the loading of > > > > > applications and > > > > > walking of paths directly on the re-export server such that > > > > > after a couple of > > > > > runs, there are practically zero packets back to the > > > > > originating NFS server > > > > > (great!). But, if we then do the same thing on a client which > > > > > is mounting that > > > > > re-export server, the re-export server now starts issuing lots > > > > > of calls back to > > > > > the originating server and invalidating it's client cache > > > > > (bad!). > > > > > > > > > > I'm not exactly sure why, but the iversion of the inode gets > > > > > changed locally > > > > > (due to atime modification?) most likely via invocation of > > > > > method > > > > > inode_inc_iversion_raw. Each time it gets incremented the > > > > > following call to > > > > > validate attributes detects changes causing it to be reloaded > > > > > from the > > > > > originating server. > > > > > > > > > > > > > I'd expect the change attribute to track what's in actual inode > > > > on the > > > > "home" server. The NFS client is supposed to (mostly) keep the > > > > raw > > > > change attribute in its i_version field. > > > > > > > > The only place we call inode_inc_iversion_raw is in > > > > nfs_inode_add_request, which I don't think you'd be hitting > > > > unless you > > > > were writing to the file while holding a write delegation. > > > > > > > > What sort of server is hosting the actual data in your setup? > > > > > > We mostly use RHEL7.6 NFS servers with XFS backed filesystems and a > > > couple of (older) Netapps too. The re-export server is running the > > > latest mainline kernel(s). > > > > > > As far as I can make out, both these originating (home) server > > > types exhibit a similar (but not exactly the same) effect on the > > > Linux NFS client cache when it is being re-exported and accessed by > > > other clients. I can replicate it when only using a read-only mount > > > at every hop so I don't think that writes are related. > > > > > > Our RHEL7 NFS servers actually mount XFS with noatime too so any > > > atime updates that might be causing this client invalidation (which > > > is what I initially thought) are ultimately a wasted effort. > > > > > > > Ok. I suspect there is a bug here somewhere, but with such a > > complicated > > setup though it's not clear to me where that bug would be though. You > > might need to do some packet sniffing and look at what the servers > > are > > sending for change attributes. > > > > nfsd4_change_attribute does mix in the ctime, so your hunch about the > > atime may be correct. atime updates imply a ctime update and that > > could > > cause nfsd to continually send a new one, even on files that aren't > > being changed. > > No. Ordinary atime updates due to read() do not trigger a ctime or > change attribute update. Only an explicit atime update through, e.g. a > call to utimensat() will do that. > Oh, interesting. I didn't realize that. > > It might be interesting to doctor nfsd4_change_attribute() to not mix > > in > > the ctime and see whether that improves things. If it does, then we > > may > > want to teach nfsd how to avoid doing that for certain types of > > filesystems. > > NACK. That would cause very incorrect behaviour for the change > attribute. It is supposed to change in all circumstances where you > ordinarily see a ctime change. I wasn't suggesting this as a real fix, just as a way to see whether we understand the problem correctly. I doubt the reexporting machine would be bumping the change_attr on its own, and this may tell you whether it's the "home" server changing it. There are other ways to determine it too though (packet sniffer, for instance). -- Jeff Layton <jlayton@xxxxxxxxxx>
