linux-kernel - Re: Linux 3.0+ Disk performance problem

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Message-ID: <CABC96KznmDJHp5gfx1YsfX9hdXGFA3buaY31P2dDy1jJQgga7w@mail.gmail.com>
Date:	Wed, 6 Mar 2013 20:05:58 +0530
From:	Harshana Ranmuthu <hranmuthu@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: Linux 3.0+ Disk performance problem - wrong pdflush behaviour

>   I don't know how to fix the issue without reverting the patch. Sorry.

with reference to post URL
http://marc.info/?l=linux-fsdevel&m=134997043820759&w=2

I was going through this post, as we are also having problems with the
same commit. In our case, we are appending to a file rather than
updating.

Let me explain the issue as I understand and the solution I think
which can fix the issue.

If a write() writes to a page (for the second time) which is being
flushed (because the page was dirty from the fist write), data written
to disk may have part from the first write and  rest from the second
write. Although, after the second write, page will have the correct
data, but disk will have corrupted data, ie, part from first write and
rest from the second write. After flush(), dirty flag on the page will
be cleared. If there has been a third write on the same page (this
time assuming a clean write), then accurate data will be written to
disk on the next flush. If there's no third write, then file on disk
will remain to have corrupted data. If we can make sure, after the
second write(), page remains dirty (because flush and write() happened
at the same time), then even there's no third write, that page will be
flushed again with accurate data, correcting the corruption on the
disk.
In summery, solution is to make sure the conflicting (write and flush
happen at the same time) pages kept dirty after the flush. This will
make sure these pages will be flushed again even there's no subsequent
write()s to them.

I don't know how easy / difficult this change is. Hope you'll consider this.

Harshana
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