Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

[Bodo Eggert]

Kazuo Ito <ito.kazuo@oss.ntt.co.jp> wrote:

> Write throughput to LVM snapshot origin volume is an order
> of magnitude slower than those to LV without snapshots or
> snapshot target volumes, especially in the case of sequential
> writes with O_SYNC on.
> 
> The following patch originally written by Kevin Jamieson and
> Jan Blunck and slightly modified for the current RCs by myself
> tries to improve the performance by modifying the behaviour
> of kcopyd, so that it pushes back an I/O job to the head of
> the job queue instead of the tail as process_jobs() currently
> does when it has to wait for free pages. This way, write
> requests aren't shuffled to cause extra seeks.

Did you check for starvation problems, too?

Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

[Kazuo Ito]

Hi,

Bodo Eggert wrote:
> Kazuo Ito <ito.kazuo@oss.ntt.co.jp> wrote:
> 
>> Write throughput to LVM snapshot origin volume is an order
>> of magnitude slower than those to LV without snapshots or
>> snapshot target volumes, especially in the case of sequential
>> writes with O_SYNC on.
>>
>> The following patch originally written by Kevin Jamieson and
>> Jan Blunck and slightly modified for the current RCs by myself
>> tries to improve the performance by modifying the behaviour
>> of kcopyd, so that it pushes back an I/O job to the head of
>> the job queue instead of the tail as process_jobs() currently
>> does when it has to wait for free pages. This way, write
>> requests aren't shuffled to cause extra seeks.
> 
> Did you check for starvation problems, too?

I ran sadc along with the tests and figures like
%memused, kbbuffers and kbcached didn't change much
before and after the patch.  And since the buffered I/O
results didn't deteriorate at least in the cases of
10 and 100 megabyte sequential writes, I assumed
there should be plentiful of memory for the kernel
to use at least in these cases.

Anyway I can send you more detailed info upon your request.

Regards,

Kazuo Ito, NTT Open Source Software Center
Phone: +81-3-5860-5125 / FAX: +81-3-5463-5690 / E-mail: ito.kazuo@oss.ntt.co.jp

Re: [dm-devel] Re: [PATCH] dm-snapshot: poor copy-on-write performance due to I/O reordering

[Kazuo Ito]

Hello,

 > Bodo Eggert wrote:
 >> Kazuo Ito <ito.kazuo@oss.ntt.co.jp> wrote:
 >>
 >>> Write throughput to LVM snapshot origin volume is an order
 >>> of magnitude slower than those to LV without snapshots or
 >>> snapshot target volumes, especially in the case of sequential
 >>> writes with O_SYNC on.
 >>>
 >>> The following patch originally written by Kevin Jamieson and
 >>> Jan Blunck and slightly modified for the current RCs by myself
 >>> tries to improve the performance by modifying the behaviour
 >>> of kcopyd, so that it pushes back an I/O job to the head of
 >>> the job queue instead of the tail as process_jobs() currently
 >>> does when it has to wait for free pages. This way, write
 >>> requests aren't shuffled to cause extra seeks.
 >>
 >> Did you check for starvation problems, too?

I have twice and four times as many pages allocated
to each kcopyd client without patching the queuing behaviour
and got these figures (MiB/s) -- allocating more buffers doesn't
seem to help much, so I don't think it's memory shortage
that matters here.

test \ # of buffer pages   256(default)      512        1024
  10M dd+fsync, create        16.00          18.60      18.95
  10M dd+fsync, update        16.14          18.39      19.70
100M dd+fsync, create        15.18          18.80      19.29
100M dd+fsync, update        15.28          19.29      19.45

--
> Kazuo Ito, NTT Open Source Software Center
> Phone: +81-3-5860-5125 / FAX: +81-3-5463-5690 / E-mail: 
> ito.kazuo@oss.ntt.co.jp