Re: [RFC v2] ext4: Don't send extra barrier during fsync if thereare no dirty pages.

From: Ric Wheeler
Date: Fri Aug 06 2010 - 06:18:02 EST

On 08/06/2010 03:04 AM, Darrick J. Wong wrote:
On Thu, Aug 05, 2010 at 12:45:04PM -0400, Ted Ts'o wrote:
P.S. If it wasn't clear, I'm still in favor of trying to coordinate
barriers across the whole file system, since that is much more likely
to help use cases that arise in real life.
Ok. I have a rough sketch of a patch to do that, and I was going to send it
out today, but the test machine caught on fire while I was hammering it with
the fsync tests one last time and ... yeah. I'm fairly sure the patch didn't
cause the fire, but I'll check anyway after I finish cleaning up.

"[PATCH] ext4: Don't set my machine ablaze with barrier requests" :P

(The patch did seem to cut barrier requests counts by about 20% though the
impact on performance was pretty small.)


Just a note, one thing that we have been doing is trying to get a reasonable regression test in place for testing data integrity. That might be useful to share as we float patches around barrier changes.

Basic test:

(1) Get a box with an external e-sata (or USB) connected drive

(2) Fire off some large load on that drive (Chris Mason had one, some of our QE engineers have been using fs_mark (fs_mark -d /your_fs/test_dir -S 0 -t 8 -F)

(3) Pull the power cable to that external box.

Of course, you can use any system and drop power, but the above setup will make sure that we kill the write cache on the device without letting the firmware destage the cache contents.

The test passes if you can now do the following:

(1) Mount the file system without error

(2) Unmount and force an fsck - that should run without reporting errors as well.

Note that the above does not use fsync in the testing.



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