[CentOS] interesting kvm lvm collision issue

Ross Walker rswwalker at gmail.com
Tue Oct 26 09:48:12 EDT 2010


On Oct 25, 2010, at 3:31 PM, Iain Morris <iain.t.morris at gmail.com> wrote:

> I've been running into a reproducible problem when using default LVM volume group names to present block devices for virtual machines in KVM, and I'm wondering why it is happening.
> 
> On dom0 I make a default VolGroup00 for the operating system.  I make a second VolGroup01 for logical volumes that will be block devices for virtual systems.
> 
> In VolGroup01, I make two lv's for one system: lv.sys1, and lv.sys1-data.  
> 
> I then build a new virtual machine called sys1, using lv.sys1 for the root filesystem, and lv.sys1-data for an independent data partition.  Everything works great after installation, and vgdisplay on both systems looks great.
> 
> If I then run vgscan, however, on the host system, it picks up the VolGroup01 I created _within_ the virtual machine, so I now have 2 VolGroup01's with different UUIDs showing up on dom0.
> 
> Now I can see how vgscan would mistakenly see the VolGroup01 of sys1 on the block device lv.sys1-data, but why are the VolGroup00 vg's not colliding as well?
> 
> When a pvdisplay is run, I have a new "physical volume" that is actually just a logical volume of the original VolGroup01:
> 
> [root at iain2 ~]# pvdisplay
>   WARNING: Duplicate VG name VolGroup01: Existing FNiKc9-BB3t-ziMg-prWW-n8RA-OMzk-obiKnf (created here) takes precedence over C8fNMV-aeSW-syIn-fWJZ-vJdK-N0As-Itrvfi
>   WARNING: Duplicate VG name VolGroup01: Existing FNiKc9-BB3t-ziMg-prWW-n8RA-OMzk-obiKnf (created here) takes precedence over C8fNMV-aeSW-syIn-fWJZ-vJdK-N0As-Itrvfi
>   --- Physical volume ---
>   PV Name               /dev/VolGroup01/lv-sys1-data
>   VG Name               VolGroup01
>   PV Size               40.00 GB / not usable 4.00 MB
>   Allocatable           yes (but full)
>   PE Size (KByte)       4096
>   Total PE              10239
>   Free PE               0
>   Allocated PE          10239
>   PV UUID               FTA4QU-ydZ7-e2Yy-nBsi-t4st-3jj7-IAkQH8
>    
>   --- Physical volume ---
>   PV Name               /dev/sda3
>   VG Name               VolGroup00
>   PV Size               39.06 GB / not usable 29.77 MB
>   Allocatable           yes (but full)
>   PE Size (KByte)       32768
>   Total PE              1249
>   Free PE               0
>   Allocated PE          1249
>   PV UUID               tTViks-3lBM-HGzV-mnN9-zRsT-fFT0-ZsJRse
>    
>   --- Physical volume ---
>   PV Name               /dev/sda2
>   VG Name               VolGroup01
>   PV Size               240.31 GB / not usable 25.75 MB
>   Allocatable           yes 
>   PE Size (KByte)       32768
>   Total PE              7689
>   Free PE               5129
>   Allocated PE          2560
>   PV UUID               ZE5Io3-WYIO-EfOQ-h03q-zGdF-Frpa-tm63fX
> 
> 
> 
> Has anyone experienced this?  It's very unnerving to know your data is intact as you add new logical volumes for kvm systems.  I suppose the lesson learned here is to provide VGs with specific host names.

You need to exclude the LVs in the host VG from being scanned for sub-VGs. It's actually easier to just list what SHOULD be scanned rather than what shouldn't.

Look in /etc/lvm/lvm.conf

You can also avoid this by creating partition based PVs in the VMs rather then whole disk PVs which would need kpartx run on the host LV before LVM could scan it.

-Ross



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