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

[Paolo Bonzini]

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.

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.