Re: [Qemu-devel] [PATCH 1.8 3/6] qemu-img: add option to specify alternate iobuffer size

[Paolo Bonzini]

Il 25/11/2013 16:07, Peter Lieven ha scritto:
>>> since the convert process is basically a sync operation it might
>>> be benificial in some case to change the hardcoded I/O buffer
>>> size to an alternate (greater) value.
>> Do you really need the extra knob?  You can just add to BlockLimits the
>> optimal transfer length, and use it unconditionally.
> If you say patch 5 and 3 are ok. What could be done is to remove
> this knob and increase the iobuf_size to cluster_size if cluster_size
> is greater.

Yes, that makes sense because it avoids unnecessary COW.

> I do not want to increase the default iobuf size to anything
> greater than 2MB. I do not know why this was choosen, but maybe
> there was a reason for it.

I think it is fine to increase it to the cluster_size or even to the
optimal transfer length (new BlockLimits field).

> The storages we use here have a very strange page size of 15MB.
> If I sent aligned 15MB chunks the performace is about +50% compared
> to the original qemu-img.

I think composing the optimal transfer length and optimal unmap
granularity (cluster_size) will work well:

* clamp maximum size to optimal transfer length

* then, round final sector down to unmap granularity

Paolo