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From: Markus Armbruster <armbru@redhat.com>
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Date: Thu, 28 Aug 2014 11:52:55 +0200
In-Reply-To: <20140828093947.GA11352@T430.redhat.com> (Fam Zheng's message of
	"Thu, 28 Aug 2014 17:39:47 +0800")
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Subject: Re: [Qemu-devel] [PATCH v2] block: Introduce "null" driver
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To: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, benoit.canet@irqsave.net, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

Fam Zheng <famz@redhat.com> writes:

> On Thu, 08/28 10:22, Markus Armbruster wrote:
>> Fam Zheng <famz@redhat.com> writes:
>> 
>> > This is an analogue to Linux null_blk. It can be used for testing block
>> > device emulation and general block layer functionalities such as
>> > coroutines and throttling, where disk IO is not necessary or wanted.
>> >
>> > Use null:// for AIO version, and null-co:// for coroutine version.
>> >
>> > Signed-off-by: Fam Zheng <famz@redhat.com>
>> >
>> > ---
>> > V2: Don't #ifdef code, add two drivers. (Benoit)
>> >     Add to QAPI BlockdevOptions. (Eric)
>> >     Add "file.size" option to override backend size. (What is a better
>> >     way to associate /dev/vd{a,b,c} with command line devices, if sizes
>> >     are the same?)
>> 
>> Is this a request for advice?  If yes, please explain the problem in
>> more detail, because I'm too dense to get it :)
>> 
>
> Yes :)
>
> I have more than one virtio-blk:
>
> -drive file=/dev/ram0,id=d0,if=none -device
> virtio-blk-pci,drive=d0,x-datap-plane=on \
> -drive file=/dev/ram1,id=d0,if=none -device virtio-blk-pci,drive=d1
>
> Then I get /dev/vda and /dev/vdb in guest. They are in the same size.
>
> The question is how do I tell which is /dev/ram0 and which is /dev/ram1,
> without bothering peaking PCI addr, etc.?

Set virtio-blk-pci's serial property to a suitable value.  The serial
number is visible in a Linux guest as
/sys/bus/virtio/devices/virtio0/block/vda/serial or similar.