Re: Ext4 performance regression: Post 2.6.30

[Greg Freemyer <greg.freemyer@gmail.com>]

On Wed, Mar 31, 2010 at 6:02 PM, Keith Mannthey <kmannth@us.ibm.com> wrote:
> On Tue, 2010-03-30 at 23:06 -0500, Eric Sandeen wrote:
>> Keith Mannthey wrote:
>> > On Mon, 2010-03-29 at 11:10 -0400, Greg Freemyer wrote:
>> >> On Mon, Mar 29, 2010 at 2:25 AM, Keith Mannthey <kmannth@us.ibm.com> wrote:
>> >>>
>> >>> After 2.6.30 I am seeing large performance regressions on a raid setup.
>> >>> I am working to publish a larger amount of data but I wanted to get some
>> >>> quick data out about what I am seeing.
>> >>>
>> >> Is mdraid involved?
>> >>
>> >> They added barrier support for some configs after 2.6.30 I believe.
>> >> It can cause a drastic perf change, but it increases reliability and
>> >> is "correct".
>> >
>> > lvm and device mapper are is involved.  The git bisect just took me to:
>> >
>> > 374bf7e7f6cc38b0483351a2029a97910eadde1b is first bad commit
>> > commit 374bf7e7f6cc38b0483351a2029a97910eadde1b
>> > Author: Mikulas Patocka <mpatocka@redhat.com>
>> > Date:   Mon Jun 22 10:12:22 2009 +0100
>> >
>> >     dm: stripe support flush
>> >
>> >     Flush support for the stripe target.
>> >
>> >     This sets ti->num_flush_requests to the number of stripes and
>> >     remaps individual flush requests to the appropriate stripe devices.
>> >
>> >     Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
>> >     Signed-off-by: Alasdair G Kergon <agk@redhat.com>
>> >
>> > :040000 040000 542f4b9b442d1371c6534f333b7e00714ef98609 d490479b660139fc1b6b0ecd17bb58c9e00e597e M  drivers
>> >
>> >
>> > This may be correct behavior but the performance penalty in this test
>> > case is pretty high.
>> >
>> > I am going to move back to current kernels and starting looking into
>> > ext4/dm flushing.
>>
>> It would probably be interesting to do a mount -o nobarrier to see if
>> that makes the regression go away.
>
> -o nobarrier takes the regression away with 2.6.34-rc3:
>
> Default mount: ~27500
>
> -o nobarrier: ~12500
>
> Barriers on this setup cost ALOT during writes.
>
> Interestingly as well the "mailserver" workload regression is also
> removed by mounting with "-o nobarrier".
>
> I am going to see what impact is seen on a single disk setup.
>
> Thanks,
>  Keith Mannthey
>  LTC FS-Dev

I'm curious if your using an internal or external journal?

I'd guess the cost of barriers is much greater with an internal
journal, but I don't recall seeing any benchmarks one way or the
other.

Greg
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