Re: Ext4 performance regression: Post 2.6.30

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

On Wed, Mar 31, 2010 at 6:14 PM, Keith Mannthey <kmannth@us.ibm.com> wrote:
> On Wed, 2010-03-31 at 18:06 -0400, Greg Freemyer wrote:
>> 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 am unsure.  How do I tell?  I am using defaults except with the -o
> nobarrier.   I know jdb2 is being used.
>
> Thanks,
>  Keith

The default is internal.  External requires a separate partition be
provided to hold the journal.

Since journals are typically very small relative to the overall
filesystem, a small raid 1 partition would be my production
recommendation to hold an external journal.

But for performance testing purposes, if you have a drive that is not
participating in your current raid setup, you can simply create a
small partition on it and use it to hold the external journal.  I
believe you can convert your existing file system to an external
journal easily and without having to recreate your file system.

Greg
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