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Date: Mon, 08 Sep 2014 17:18:47 +0200
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized
 read/write requests
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To: Paolo Bonzini <pbonzini@redhat.com>, ronnie sahlberg <ronniesahlberg@gmail.com>
Cc: =?UTF-8?B?QmVub8OudCBDYW5ldA==?= <benoit.canet@irqsave.net>, Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Max Reitz <mreitz@redhat.com>

On 08.09.2014 17:15, Paolo Bonzini wrote:
> Il 08/09/2014 17:13, ronnie sahlberg ha scritto:
>> What I would like to see would also be to report these limitations to
>> the guest itself to prevent it from generating too large I/Os.
> That's difficult because you don't want a backend change (e.g. from
> local storage to iSCSI) to change the vital product data in the guest.

I hadn't that in mind....

>
> That's why we have splitting code for discard, and why we would have to
> add it for read/write too.

Why should a guest generate such big requests. Afaik the reported
limit for e.g. virtio-blk is 1024 sectors (reported through blockdev --getmaxsect /dev/vda).
I think it was only the multiwrite_merge code causing trouble here.

Peter