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Date: Wed, 10 Oct 2012 11:01:31 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Use SCSI command to get size of SG device
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To: Chen HanXiao <chenhanxiao@cn.fujitsu.com>
Cc: 'Christoph Hellwig' <hch@lst.de>, qemu-devel@nongnu.org

Il 10/10/2012 10:33, Chen HanXiao ha scritto:
>> You need to use scsi-block instead of scsi-generic.  However, I don't see
>> how this can work.  After the snapshot, the image will be qcow2, not raw, and
>> thus it will not support bdrv_aio_ioctl.  Hence any SCSI command (for
>> scsi-generic) or any non-data SCSI command (for scsi-block) will fail.
>
> That's the issue what I also encountered.
> Do you mean that it is impossible for us to enable snapshot for
> scsi-generic?

It's impossible, and rightly so: the user could anyway bypass the
snapshotting and write to the disk (WRITE SAME for example).

Not just snapshot=on, which fails because it is early enough that the
disk fails to initialize.  Live snapshots also ought to be blocked, it
is a bug that they are not.

Paolo