Re: [Qemu-devel] [PATCH v6 0/3] linux-aio: introduce submit I/O as a batch

["Zhang Haoyu" <zhanghy@sangfor.com>]

>>> Hi,
>>> 
>>> The commit 580b6b2aa2(dataplane: use the QEMU block layer for I/O)
>>> introduces ~40% throughput regression on virtio-blk dataplane, and
>>> one of causes is that submitting I/O as a batch is removed.
>>> 
>>> This patchset trys to introduce this mechanism on block, at least,
>>> linux-aio can benefit from that.
>>> 
>>> With these patches, it is observed that thoughout on virtio-blk
>>> dataplane can be improved a lot, see data in commit log of patch
>>> 3/3.
>>> 
>>> It should be possible to apply the batch mechanism to other devices
>>> (such as virtio-scsi) too.
>>> 
>>> TODO:
>>> 	- support queuing I/O to multi files for scsi devies, which
>>>     need some changes to linux-aio
>>> 
>>> V6:
>>> 	- fix requests leak if part of them arn't submitted successfully,
>>> 	pointed by Stefan
>>> 	- linux-aio.c coding style fix
>>> 
>>> V5:
>>> 	- rebase on v2.1.0-rc0 of qemu.git/master
>>> 	- block/linux-aio.c code style fix
>>> 	- don't flush io queue before flush, pointed by Paolo
>>> 
>>> V4:
>>> 	- support other non-raw formats with under-optimized performance
>>> 	- use reference counter for plug & unplug
>>> 	- flush io queue before sending flush command
>>> 
>>> V3:
>>> 	- only support submitting I/O as a batch for raw format, pointed by
>>>     Kevin
>>> 
>>> V2:
>>> 	- define return value of bdrv_io_unplug as void, suggested by Paolo
>>> 	- avoid busy-wait for handling io_submit
>>> V1:
>>> 	- move queuing io stuff into linux-aio.c as suggested by Paolo
>>
>>Thanks, applied to my block tree:
>>https://github.com/stefanha/qemu/commits/block
>>
>Can we use the queued io data as caches, 
>io write will directly return and tell the guest the io is completed after the io is enqueued, 
>better user experience for burst io,
>and io-read will firstly search the io queue, if matched data found, directly get the data from the queue, 
>if not, then read the data from the disk or host page cache.

Because host page cache cannot assure the order of io-write, leading to VM image data corruption if suddenly poweroff happened,
but we can assure the order of io-write in queue, and use cache=none/writethrough/directsync to submit the batched IO to disk directly, 
bypass host page cache.
IO merging also can be performed in the queue.

>Any ideas?
>
>Thanks,
>Zhang Haoyu
>
>>In Patch 2 we should complete requests with -EIO if io_submit() returned
>>0 <= ret < len.  I fixed this up when applying because the patch was
>>completing with a bogus ret value.
>>
>>Stefan