Re: [Qemu-devel] 4k seq read splitting for virtio-blk - possible workarounds?

[Paolo Bonzini]

On 26/10/2015 17:31, Andrey Korolyov wrote:
>> the virtio block device is always splitting a single read
>> range request to 4k ones, bringing the overall performance of the
>> sequential reads far below virtio-scsi.
>> 
>> How does the blktrace look like in the guest?
> 
> Yep, thanks for suggestion. It looks now like a pure driver issue:
> 
>  Reads Queued:       11008,    44032KiB  Writes Queued:           0,        
> 0KiB
>  Read Dispatches:    11008,    44032KiB  Write Dispatches:        0,        
> 0KiB
> 
> vs
> 
>  Reads Queued:      185728,   742912KiB  Writes Queued:           0,        
> 0KiB
>  Read Dispatches:     2902,   742912KiB  Write Dispatches:        0,        
> 0KiB
> 
> Because guest virtio-blk driver lacks *any* blk scheduler management,
> this is kinda logical. Requests for scsi backend are dispatched in
                                                       ^^^^^^^^^^

queued you mean?

> single block-sized chunks as well, but they are mostly merged by a
> scheduler before being passed to the device layer. Could be there any
> improvements over the situation except writing an underlay b/w virtio
> emulator backend and the real storage?

This is probably the fall-out of converting the virtio-blk to use
blk-mq, which was premature to say the least.  Jeff Moyer was working on
it, but I'm not sure if this has been merged.  Andrey, what kernel are
you using?

Paolo