Re: [Qemu-devel] [PATCH 0/3] block: Handle multiple write requests at once

[Avi Kivity]

On 09/01/2009 06:52 PM, Christoph Hellwig wrote:
> On Tue, Sep 01, 2009 at 03:51:49PM +0200, Kevin Wolf wrote:
>    
> > virtio often issues multiple requests in a row, but each one independently. If
> > the block drivers knew all of the requests, they could optimize the way they
> > handle the requests. See the description of patch 3 for how qcow2 can use this
> > to avoid unnecessary writes to the disk.
> >      
> I think this interface is extremly awkward and the layering is wrong.
>    

Can you explain why?

> Everyone benefits from having one large instead of multiple small
> requests, so if we do get multiple sequential write requests we should
> always merged it at a high level even before starting to issue AIO,
> e.g. do it all in virtio-blk.
>    

I don't think we should merge requests for the guest.  If it issued 
multiple requests, that's what it wants.

I see two benefits to multiwrite:

- handle layout issues in qcow2 (so we can allocate for all the 
requests, then submit them in parallel)
- submit multiple requests at once for linux-aio


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