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Date: Fri, 10 Aug 2012 12:30:35 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] virtio-scsi vs. virtio-blk
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To: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, Ronnie Sahlberg <ronniesahlberg@gmail.com>

Il 10/08/2012 12:28, Stefan Priebe - Profihost AG ha scritto:
> I'm using iscsi. So no raw or qcow2.

Ok, then you need to use scsi-block as your device instead of scsi-disk
or scsi-hd.  This will disable the QEMU SCSI emulation and let your VM
talk directly to the NAS.

CCing Ronnie who may be interested in bug reports since I'm on holiday
starting "soon".

Paolo

> 
> Thanks,
> 
> Stefan
> 
> Am 10.08.2012 12:20, schrieb Paolo Bonzini:
>> Il 10/08/2012 11:22, Stefan Priebe - Profihost AG ha scritto:
>>> virtio-scsi is now working fine. Could you please help me to get discard
>>> / trim running? I can't find any information what is needed to get
>>> discard / trim working.
>>
>> You need to add discard_granularity=NNN, where NNN is usually 512 for
>> raw and the cluster size (65536) for qcow2.
>>
>> However, discard is supported only for XFS on raw, and on qcow2 it will
>> not reclaim space---the space will only be reused for future qcow2
>> allocation.
>>
>> Better support for discard on raw (other filesystems + block devices),
>> and more efficient support also on other formats is on my todo list for
>> the future.  However, an efficient implementation unfortunately requires
>> changes at all levels including the kernel.
>>
>> I hope to present something about it at KVM Forum.
>>
>> Paolo
>>