[PATCH 0/4] cifs: pave way for change to "strictcache" by default

Steve French smfrench at gmail.com
Mon Apr 30 09:11:43 MDT 2012

There is a related question of how deferring writes slightly (e.g. to
buffer a full page or a full smb buffer) could corrupt data?

AFAIK - there isn't an ordering guarantee in posix when writes from
two different applications overlap (thus the need for byte range locks
or other synchronization mechanisms, or flush/fsync).

On Mon, Apr 30, 2012 at 6:28 AM, Jeff Layton <jlayton at samba.org> wrote:
> On Fri, 27 Apr 2012 06:56:15 -0400
> Jeff Layton <jlayton at redhat.com> wrote:
>> cifs.ko currently does not follow the CIFS protocol as it should by
>> default. The protocol specifies (paraphrasing) "Thou shalt not cache
>> file data without an oplock." cifs.ko by default *does* do this however,
>> opting for a more NFS-like cache coherency scheme. It caches file data
>> in the pagecache, and invalidate it when the attributes look like the
>> file has changed.
>> Unfortunately, this can and does cause cache coherency problems. The
>> kernel can end up buffering up writes for longer than it should. In
>> particular, internal testing at RH with Samba/CTDB has shown cache
>> coherency problems when we don't mount on the clients with
>> strictcache:
>>     https://bugzilla.redhat.com/show_bug.cgi?id=742314
>> Up until now, we've shied away from changing this default primarily for
>> performance reasons. The code to handle uncached reads and writes was
>> synchronous, and very slow. With the addition of async uncached writes
>> in 3.4 and async uncached reads in 3.5, I think performance is good
>> enough that we ought to change the defaults to be "strictcache" for
>> better adherence to the protocol.
>> This patchset begins this transition by cleaning up the mount options
>> that we use to control the cache behavior, and adding some warnings
>> about the impending behavior change. I'm proposing that we follow the
>> kernel convention of warning for 2 releases about user-visible changes
>> and then we'll make the switch in 3.7.
>> At the same time, it corrects a problem with the current code. The
>> different cache behaviors are not considered to be mutually exclusive
>> even though they should be. If someone (for instance) specifies
>> "strictcache" and "fsc" the results are poorly defined. The first
>> patch corrects this.
>> If this looks acceptable, I'll also plan to spin up a manpage patch
>> to help document this change.
> (cc'ing samba-technical too for those that don't follow linux-cifs...)
> I discussed this some via IM with Steve last week and he has his doubts
> about whether we should do this. I think we ought to bring this
> discussion to the lists though so we can try to settle this in time for
> the next kernel merge window.
> The (valid) point that he brings up is that this will harm performance
> when an application is doing small I/Os sizes and does not have an
> oplock. This is true -- reads and writes will be synchronous and will
> match the sizes that come from userspace. An application doing small
> I/Os will suffer performance-wise when it does not have an oplock on an
> open file.
> He also claims that Windows will still buffer up writes when it doesn't
> have an oplock in an effort to do larger I/Os on the wire. I've got a
> dochelp request into MS to clarify this point and what the protocol
> mandates, but I don't think the answer there will have much bearing on
> what we should do. There are a number of problems with keeping
> "cache=loose" the default...
> When we do I/O out of the pagecache, then we more or less do it along
> page boundaries. If another client has locked a region in the middle of
> a page, then that request will generally fail. We have no way to know
> what region is locked, so we have no way to handle this case.
> We also have quite a few documented cases where using "strictcache" has
> fixed cache coherency problems. Here's one, for instance, but there are
> others:
>    https://bugzilla.samba.org/show_bug.cgi?id=6174
> I think it's important that cifs.ko is safe to use by default, even if
> it's a little slower. We can always tell people "try cache=loose" if
> their workload is slow with "cache=strict", but having defaults that
> can munch your data is really not acceptable.
> I guess the question we have to answer is:
> "Is it worth risking data corruption in order to eke out better
> performance when a user is doing small I/Os and does not have an oplock?"
> To me, the answer is an unqualified "no". In most cases, you don't have
> an oplock because another client has the file open. In those cases,
> cache coherency generally trumps performance. In cases where it
> doesn't, a user has the freedom to mount with "cache=loose" with the
> understanding of all that that entails.
> Thoughts?
> --
> Jeff Layton <jlayton at samba.org>



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