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Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxA

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxAioState within an AioContext
Date: Wed, 11 May 2016 14:22:11 +0100
User-agent: Mutt/1.6.0 (2016-04-01)

On Tue, May 10, 2016 at 12:32:27PM +0200, Paolo Bonzini wrote:
> On 10/05/2016 11:40, Kevin Wolf wrote:
> > > Regarding performance, I'm thinking about a guest with 8 disks (queue
> > > depth 32).  The worst case is when the guest submits 32 requests at once
> > > but the Linux AIO event limit has already been reached.  Then the disk
> > > is starved until other disks' requests complete.
> >
> > Sounds like a valid concern.
> Oh, so you're concerned about the non-dataplane case.  My suspicion is
> that, with such a number of outstanding I/O, you probably have bad
> performance unless you use dataplane.
> Also, aio=threads has had a 64 I/O limit for years and we've never heard
> about problems with stuck I/O.  But I agree that it's one area to keep
> an eye on.

Good point.  In that case I don't feel so bad if we merge this.


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