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From: Trond Myklebust <trondmy@hammerspace.com>
To: "jlayton@kernel.org" <jlayton@kernel.org>,
	"daire@dneg.com" <daire@dneg.com>
Cc: "linux-cachefs@redhat.com" <linux-cachefs@redhat.com>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [Linux-cachefs] Adventures in NFS re-exporting
Date: Thu, 1 Oct 2020 12:38:34 +0000	[thread overview]
Message-ID: <7cdb496a2b77dd62b8e6373c28926f11a4816d49.camel@hammerspace.com> (raw)
In-Reply-To: <3243730b0661de0ac0864a9bb5375f894b266220.camel@kernel.org>

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@kernel.org
> > 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@dneg.com
> > > > 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.

> 
> 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.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com



  reply	other threads:[~2020-10-01 12:39 UTC|newest]

Thread overview: 129+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-07 17:31 Adventures in NFS re-exporting Daire Byrne
2020-09-08  9:40 ` Mkrtchyan, Tigran
2020-09-08 11:06   ` Daire Byrne
2020-09-15 17:21 ` J. Bruce Fields
2020-09-15 19:59   ` Trond Myklebust
2020-09-16 16:01     ` Daire Byrne
2020-10-19 16:19       ` Daire Byrne
2020-10-19 17:53         ` [PATCH 0/2] Add NFSv3 emulation of the lookupp operation trondmy
2020-10-19 17:53           ` [PATCH 1/2] NFSv3: Refactor nfs3_proc_lookup() to split out the dentry trondmy
2020-10-19 17:53             ` [PATCH 2/2] NFSv3: Add emulation of the lookupp() operation trondmy
2020-10-19 20:05         ` [PATCH v2 0/2] Add NFSv3 emulation of the lookupp operation trondmy
2020-10-19 20:05           ` [PATCH v2 1/2] NFSv3: Refactor nfs3_proc_lookup() to split out the dentry trondmy
2020-10-19 20:05             ` [PATCH v2 2/2] NFSv3: Add emulation of the lookupp() operation trondmy
2020-10-20 18:37         ` [PATCH v3 0/3] Add NFSv3 emulation of the lookupp operation trondmy
2020-10-20 18:37           ` [PATCH v3 1/3] NFSv3: Refactor nfs3_proc_lookup() to split out the dentry trondmy
2020-10-20 18:37             ` [PATCH v3 2/3] NFSv3: Add emulation of the lookupp() operation trondmy
2020-10-20 18:37               ` [PATCH v3 3/3] NFSv4: Observe the NFS_MOUNT_SOFTREVAL flag in _nfs4_proc_lookupp trondmy
2020-10-21  9:33         ` Adventures in NFS re-exporting Daire Byrne
2020-11-09 16:02           ` bfields
2020-11-12 13:01             ` Daire Byrne
2020-11-12 13:57               ` bfields
2020-11-12 18:33                 ` Daire Byrne
2020-11-12 20:55                   ` bfields
2020-11-12 23:05                     ` Daire Byrne
2020-11-13 14:50                       ` bfields
2020-11-13 22:26                         ` bfields
2020-11-14 12:57                           ` Daire Byrne
2020-11-16 15:18                             ` bfields
2020-11-16 15:53                             ` bfields
2020-11-16 19:21                               ` Daire Byrne
2020-11-16 15:29                           ` Jeff Layton
2020-11-16 15:56                             ` bfields
2020-11-16 16:03                               ` Jeff Layton
2020-11-16 16:14                                 ` bfields
2020-11-16 16:38                                   ` Jeff Layton
2020-11-16 19:03                                     ` bfields
2020-11-16 20:03                                       ` Jeff Layton
2020-11-17  3:16                                         ` bfields
2020-11-17  3:18                                           ` [PATCH 1/4] nfsd: move fill_{pre,post}_wcc to nfsfh.c J. Bruce Fields
2020-11-17  3:18                                             ` [PATCH 2/4] nfsd: pre/post attr is using wrong change attribute J. Bruce Fields
2020-11-17 12:34                                               ` Jeff Layton
2020-11-17 15:26                                                 ` J. Bruce Fields
2020-11-17 15:34                                                   ` Jeff Layton
2020-11-20 22:38                                                     ` J. Bruce Fields
2020-11-20 22:39                                                       ` [PATCH 1/8] nfsd: only call inode_query_iversion in the I_VERSION case J. Bruce Fields
2020-11-20 22:39                                                         ` [PATCH 2/8] nfsd: simplify nfsd4_change_info J. Bruce Fields
2020-11-20 22:39                                                         ` [PATCH 3/8] nfsd: minor nfsd4_change_attribute cleanup J. Bruce Fields
2020-11-21  0:34                                                           ` Jeff Layton
2020-11-20 22:39                                                         ` [PATCH 4/8] nfsd4: don't query change attribute in v2/v3 case J. Bruce Fields
2020-11-20 22:39                                                         ` [PATCH 5/8] nfs: use change attribute for NFS re-exports J. Bruce Fields
2020-11-20 22:39                                                         ` [PATCH 6/8] nfsd: move change attribute generation to filesystem J. Bruce Fields
2020-11-21  0:58                                                           ` Jeff Layton
2020-11-21  1:01                                                             ` J. Bruce Fields
2020-11-21 13:00                                                           ` Jeff Layton
2020-11-20 22:39                                                         ` [PATCH 7/8] nfsd: skip some unnecessary stats in the v4 case J. Bruce Fields
2020-11-20 22:39                                                         ` [PATCH 8/8] Revert "nfsd4: support change_attr_type attribute" J. Bruce Fields
2020-11-20 22:44                                                       ` [PATCH 2/4] nfsd: pre/post attr is using wrong change attribute J. Bruce Fields
2020-11-21  1:03                                                         ` Jeff Layton
2020-11-21 21:44                                                           ` Daire Byrne
2020-11-22  0:02                                                             ` bfields
2020-11-22  1:55                                                               ` Daire Byrne
2020-11-22  3:03                                                                 ` bfields
2020-11-23 20:07                                                                   ` Daire Byrne
2020-11-17 15:25                                               ` J. Bruce Fields
2020-11-17  3:18                                             ` [PATCH 3/4] nfs: don't mangle i_version on NFS J. Bruce Fields
2020-11-17 12:27                                               ` Jeff Layton
2020-11-17 14:14                                                 ` J. Bruce Fields
2020-11-17  3:18                                             ` [PATCH 4/4] nfs: support i_version in the NFSv4 case J. Bruce Fields
2020-11-17 12:34                                               ` Jeff Layton
2020-11-24 20:35               ` Adventures in NFS re-exporting Daire Byrne
2020-11-24 21:15                 ` bfields
2020-11-24 22:15                   ` Frank Filz
2020-11-25 14:47                     ` 'bfields'
2020-11-25 16:25                       ` Frank Filz
2020-11-25 19:03                         ` 'bfields'
2020-11-26  0:04                           ` Frank Filz
2020-11-25 17:14                   ` Daire Byrne
2020-11-25 19:31                     ` bfields
2020-12-03 12:20                     ` Daire Byrne
2020-12-03 18:51                       ` bfields
2020-12-03 20:27                         ` Trond Myklebust
2020-12-03 21:13                           ` bfields
2020-12-03 21:32                             ` Frank Filz
2020-12-03 21:34                             ` Trond Myklebust
2020-12-03 21:45                               ` Frank Filz
2020-12-03 21:57                                 ` Trond Myklebust
2020-12-03 22:04                                   ` bfields
2020-12-03 22:14                                     ` Trond Myklebust
2020-12-03 22:39                                       ` Frank Filz
2020-12-03 22:50                                         ` Trond Myklebust
2020-12-03 23:34                                           ` Frank Filz
2020-12-03 22:44                                       ` bfields
2020-12-03 21:54                               ` bfields
2020-12-03 22:45                               ` bfields
2020-12-03 22:53                                 ` Trond Myklebust
2020-12-03 23:16                                   ` bfields
2020-12-03 23:28                                     ` Frank Filz
2020-12-04  1:02                                     ` Trond Myklebust
2020-12-04  1:41                                       ` bfields
2020-12-04  2:27                                         ` Trond Myklebust
2020-09-17 16:01   ` Daire Byrne
2020-09-17 19:09     ` bfields
2020-09-17 20:23       ` Frank van der Linden
2020-09-17 21:57         ` bfields
2020-09-19 11:08           ` Daire Byrne
2020-09-22 16:43         ` Chuck Lever
2020-09-23 20:25           ` Daire Byrne
2020-09-23 21:01             ` Frank van der Linden
2020-09-26  9:00               ` Daire Byrne
2020-09-28 15:49                 ` Frank van der Linden
2020-09-28 16:08                   ` Chuck Lever
2020-09-28 17:42                     ` Frank van der Linden
2020-09-22 12:31 ` Daire Byrne
2020-09-22 13:52   ` Trond Myklebust
2020-09-23 12:40     ` J. Bruce Fields
2020-09-23 13:09       ` Trond Myklebust
2020-09-23 17:07         ` bfields
2020-09-30 19:30   ` [Linux-cachefs] " Jeff Layton
2020-10-01  0:09     ` Daire Byrne
2020-10-01 10:36       ` Jeff Layton
2020-10-01 12:38         ` Trond Myklebust [this message]
2020-10-01 16:39           ` Jeff Layton
2020-10-05 12:54         ` Daire Byrne
2020-10-13  9:59           ` Daire Byrne
2020-10-01 18:41     ` J. Bruce Fields
2020-10-01 19:24       ` Trond Myklebust
2020-10-01 19:26         ` bfields
2020-10-01 19:29           ` Trond Myklebust
2020-10-01 19:51             ` bfields

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