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Re: [Qemu-devel] Re: [PATCH 08/12] block: Catch attempt to attach multip


From: Markus Armbruster
Subject: Re: [Qemu-devel] Re: [PATCH 08/12] block: Catch attempt to attach multiple devices to a blockdev
Date: Sat, 26 Jun 2010 16:44:11 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

Christoph Hellwig <address@hidden> writes:

> On Fri, Jun 25, 2010 at 06:53:28PM +0200, Markus Armbruster wrote:
>> For instance, -device scsi-disk,drive=foo -device scsi-disk,drive=foo
>> happily creates two SCSI disks connected to the same block device.
>> It's all downhill from there.
>
> And from some quick testing a while ago the thing seems to actually
> work.  Not that I think that it is a good idea, but do we want to change
> behaviour in that respect?

Valid question.  I'd answer yes.  It's an easy error to make, and likely
to end in massive file system corruption in the guest.

>> Device usb-storage deliberately attaches twice to the same blockdev,
>> which fails with the fix in place.  Detach before the second attach
>> there.
>
> Can anyone explain what the hell usb storage is actually trying to do
> with the two drives?

It's actually a SCSI controller with a single drive on its single bus.

-device usb-storage,drive=foo creates *two* devices: usb-storage itself,
which serves as SCSI controller, and scsi-disk for the drive.
usb-storage copies its drive property to scsi-disk.

I don't like this.  Each -device should create just one device.



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