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Re: question about xfs_fsync on linux

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: question about xfs_fsync on linux
From: Chris Torek <chris.torek@xxxxxxxxxxxxx>
Date: Mon, 14 Jul 2008 19:29:16 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: Your message of "Tue, 15 Jul 2008 09:03:00 +1000." <20080714230300.GY29319@disturbed>
Sender: xfs-bounce@xxxxxxxxxxx
>What kernel(s), exactly, is/are showing this problem?

Well, that part is a bit tricky.  The base kernel is 2.6.21
but it has a lot of patches, including the one you mentioned.
(The customer is double checking to make sure they actually have
that patch in.)

>> We have a customer who is seeing data not "make it" to disk on a
>> stress test that involves doing an fsync() or fdatasync() and then
>> deliberately rebooting the machine (to simulate a failure; note
>> that the underlying RAID has its own battery backup and this is
>> just one of many different parts of the stress-test).
>
>What is the symptom? The file size does not change? The file the
>right size but has no data in it?

Their system has a large number of databases (on the order of 50)
all open simultaneously, and is using directIO (with a call to
fdatasync()) to make entries in many of them, and apparently *some*
of them get corrupted.  Exactly how, I do not know: naturally, we
cannot reproduce this with our own system, and when they tried a
simplified system with just one database the problem went away on
their end too.  (Agh.)

>No, the filemap_fdatawrite() has already been executed by this
>point [by do_fsync()].

D'oh!  I somehow missed this in eyeballing the code paths.

>However, I do ask exactly what kernel version you are running ...

It is mostly 2.6.21.  We brought in a large number of miscellaneous
XFS fixes, not including the ones that remove the "behavior" layer
stuff, but definitely including this one:

>http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;
>h=978b7237123d007b9fa983af6e0e2fa8f97f9934

(which of course necessitated a bit of hacking on the patches to
fit, as a lot of the later ones assume the bhv* layer has been
removed).

Chris


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