Re: [Qemu-devel] [PATCH v5 0/2] block: enforce minimal 4096 alignment in qemu_blockalign

[Dmitry Monakhov <dmonakhov@openvz.org>]

<#secure method=pgpmime mode=sign>

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 01/06/2015 13:16, Dmitry Monakhov wrote:
>> 259,0   31      385     0.719283423 10729  Q  WS 29376775 + 248 [qemu-io]
>> 259,0   31      388     0.719287600 10729  Q  WS 29377023 + 8 [qemu-io]
>> 259,0   31      391     0.719315193 10729  Q  WS 29377031 + 248 [qemu-io]
>> 259,0   31      394     0.719319179 10729  Q  WS 29377279 + 8 [qemu-io]
>
> Compared to the old one:
>
>>    9,0   11        1     0.000000000 11151  Q  WS 312737792 + 1023 [qemu-img]
>>    9,0   11        2     0.000007938 11151  Q  WS 312738815 + 8 [qemu-img]
>>    9,0   11        3     0.000030735 11151  Q  WS 312738823 + 1016 [qemu-img]
>>    9,0   11        4     0.000032482 11151  Q  WS 312739839 + 8 [qemu-img]
>>    9,0   11        5     0.000041379 11151  Q  WS 312739847 + 1016 [qemu-img]
>>    9,0   11        6     0.000042818 11151  Q  WS 312740863 + 8 [qemu-img]
>>    9,0   11        7     0.000051236 11151  Q  WS 312740871 + 1017 [qemu-img]
>
> I suspect the difference is that my patch avoids RMW operations on disks
> that have 512-byte logical blocks and 4K physical blocks.
Probably. I've checked bio/request submission path for my nvme device and it is
appeared that merge is merges are disabled for that device.
/sys/block/nvme0n1/queue/nomerges -> 2 (QUEUE_FLAG_NOMERGES)
so we follow this trace
->blk_mq_make_request(bio)
  if(!blk_mq_merge_queue_io)
     blk_mq_run_hw_queue(data.hctx, !is_sync || is_flush_fua);

So the only thing we can in order to improve performance is to is to
modify fs/direct-io.c -> bio submission path by constructing bios with maximum size.
>
> Paolo