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Date: Wed, 10 Oct 2012 08:33:57 +0200
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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 04:11, Chen HanXiao ha scritto:
>> > 
>> > On Tue, Oct 09, 2012 at 05:42:01PM +0800, Chen HanXiao wrote:
>>> > > When we use SCSI generic device as disk image, function lseek could
>>> > > not get the size of this kind of device.
>>> > > So try to use SCSI command Read Capacity(10) when lseek failed to get
>>> > > the size of SCSI generic device.
>> > 
>> > Eww, this is ugly as hell.  Why would you even need the size for a raw
>> > passthrough device?
>
> If we want to enable snapshot for SCSI generic device as disk image, the size
> of SCSI generic device is needed. Function lseek could not get this, SCSI
> command can finish the job.
> Only when lseek failed would Read Capacity command be sent. 

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.

Can you give an example of what exactly you are trying to do?

Paolo