linux-ext4 - Problem with ext4_sync_file in no-journal mode.

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Date:	Tue, 25 Aug 2009 10:44:05 -0700
From:	Frank Mayhar <fmayhar@...gle.com>
To:	linux-ext4@...r.kernel.org
Subject: Problem with ext4_sync_file in no-journal mode.

Our powerfail testing turned up an odd regression when using fsync() in
no-journal mode to force data to the device.  We saw loss rates (both
file and data) that were much higher than the same test using ext2 (60+%
loss versus <10%).  We've done some investigation and one thing that
stood out was that in the no-journal case, ext4_sync_file() was just
calling sync_inode() (and nothing else), while ext2_sync_file(), for
comparison, was also calling sync_mapping_buffers() to actually push the
data out.

I therefore hacked ext4_sync_file() to call sync_mapping_buffers() in
the no-journal case; when we reran the test we saw that the loss rate
dropped from 60+% to around 50%.  While it's clear that we have more
work to do in this area, this is a significant improvement.  It appears
that this was just missed when we did the no-journal work.  Do you guys
concur?

The other interesting bit of this is that ext4 no-journal without using
fsync() has, apparently, basically the same loss rate as ext2 with
fsync().
-- 
Frank Mayhar <fmayhar@...gle.com>
Google, Inc.

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