How to get the real device in guest os after attached a disk?

How to get the real device in guest os after attached a disk?

[Wangpan]

Hi all,
I have a question as the subject above, the reason I want to know this is that, if I attach some disks on the guest,
for example, I specified /dev/vdc&/dev/vdd(target device) at the cmd line by using 'virsh attach-disk', but they may be /dev/vdb&/dev/vdc in the guest os,
so if the guest user want to detach the /dev/vdb(guest device), he\she will be confused with the two target devices /dev/vdb&/dev/vdc,
because he\she doesn't know the corresponding relation of the guest device and target device, 
he\she may detach an error device /dev/vdd(target device) which corresponding to /dev/vdc(guest device).

Could anyone give me some idea?
Thanks in advance.

Best Regards
Wangpan

Re: How to get the real device in guest os after attached a disk?

[Stefan Hajnoczi]

On Tue, Jul 17, 2012 at 4:40 AM, Wangpan <hzwangpan@corp.netease.com> wrote:
> I have a question as the subject above, the reason I want to know this is that, if I attach some disks on the guest,
> for example, I specified /dev/vdc&/dev/vdd(target device) at the cmd line by using 'virsh attach-disk', but they may be /dev/vdb&/dev/vdc in the guest os,
> so if the guest user want to detach the /dev/vdb(guest device), he\she will be confused with the two target devices /dev/vdb&/dev/vdc,
> because he\she doesn't know the corresponding relation of the guest device and target device,
> he\she may detach an error device /dev/vdd(target device) which corresponding to /dev/vdc(guest device).
>
> Could anyone give me some idea?

You can use the virtio-blk serial or file system/volume labels to
distinguish them.

For libvirt disk <serial> information, see the Domain XML documentation:
http://libvirt.org/formatdomain.html#elementsDisks

Stefan

Re: Re: How to get the real device in guest os after attached a disk?

[Wangpan]

Thanks Stefan,
But how can I get the serial info in the guest os?
I have tried 'hdparm', 'sdparm', 'lshw', 'smartctl' and failed finally.
for example, 
root@debian:~# hdparm -i /dev/vdc

/dev/vdb:
 HDIO_DRIVE_CMD(identify) failed: Inappropriate ioctl for device
 HDIO_GET_IDENTITY failed: Inappropriate ioctl for device

The XML be attached is: 
<disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/dev/nbd1'/>
      <target dev='vdc' bus='virtio'/>
      <serial>WD-WMAP9A966149</serial>
      <alias name='virtio-disk2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
</disk>

The kernel info:
root@debian:~# uname -a
Linux debian 2.6.32-5-amd64 #1 SMP Mon Jan 16 16:22:28 UTC 2012 x86_64 GNU/Linux


>On Tue, Jul 17, 2012 at 4:40 AM, Wangpan <hzwangpan@corp.netease.com> wrote: 
>> I have a question as the subject above, the reason I want to know this is that, if I attach some disks on the guest, 
>> for example, I specified /dev/vdc&/dev/vdd(target device) at the cmd line by using 'virsh attach-disk', but they may be /dev/vdb&/dev/vdc in the guest os, 
>> so if the guest user want to detach the /dev/vdb(guest device), he\she will be confused with the two target devices /dev/vdb&/dev/vdc, 
>> because he\she doesn't know the corresponding relation of the guest device and target device, 
>> he\she may detach an error device /dev/vdd(target device) which corresponding to /dev/vdc(guest device). 
>> 
>> Could anyone give me some idea? 
>
>You can use the virtio-blk serial or file system/volume labels to 
>distinguish them. 
>
>For libvirt disk <serial> information, see the Domain XML documentation: 
>http://libvirt.org/formatdomain.html#elementsDisks 
>
>Stefan 

Re: Re: How to get the real device in guest os after attached a disk?

[Stefan Hajnoczi]

On Wed, Jul 18, 2012 at 5:01 AM, Wangpan <hzwangpan@corp.netease.com> wrote:
> But how can I get the serial info in the guest os?

General documentation on persistent block device naming (just grabbed
the first useful link of Google):
https://wiki.archlinux.org/index.php/Persistent_block_device_naming

If your guest is recent enough it will automatically show the
virtio-blk device in /dev/disk/by-id.

If not, you may be able to check /sys/block/vda/serial yourself.  If
that file is not present either your guest doesn't support virtio-blk
serial: either upgrade to a newer guest OS version or use one of the
other persistent naming mechanisms explained in the link above.

Stefan