dm-snapshot: poor copy-on-write performance due to I/O reordering
> Bodo Eggert wrote:
>> Kazuo Ito <firstname.lastname@example.org> 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:
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