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

[Kazuo Ito]

Hi,

Bodo Eggert wrote:
> Kazuo Ito <ito.kazuo at 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 at oss.ntt.co.jp