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Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized read/wr


From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized read/write requests
Date: Mon, 8 Sep 2014 18:18:26 +0200


> Am 08.09.2014 um 17:27 schrieb Paolo Bonzini <address@hidden>:
> 
> Il 08/09/2014 17:18, Peter Lieven ha scritto:
>>> 
>>> 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.
> 
> When copying data, gparted will try using very large I/O sizes.  Of
> course if something breaks it will just use a smaller size, but it would
> make performance worse.
> 
> I tried now (with local storage, not virtual---but with such large block
> sizes it's disk bound anyway, one request can take 0.1 seconds to
> execute) and a 2 MB block size is 20% slower than 16 MB block size on
> your usual 3.5" rotational SATA disk.
> 

can you share with what command exactly you ran these tests?

i tried myself and found that without multiwrite_merge i was not able to create 
a request bigger than 0xffff sectors from inside linux.

Peter

> Paolo
> 
>> 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.
> 
> 



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