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

[Kazuo Ito <ito kazuo oss ntt co jp>]

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