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Compute instance root disk thin provision and lvm

asked 2014-04-09 09:58:07 -0500

atze.devries gravatar image

Hi there,

Currrently we have a openstack setup with one control and two compute nodes. The compute nodes have the following nova configuration on the libvirt part.


The base images (comming from glance) are saved in /var/lib/nova/instances/_base as a file on de compute node. Once a instance is started the rootdisk is a lvm volume with 100% of it's size.

We would like to have thin provisioned rootdisks. Is there a way to configure this with lvm? Or should we ditch the lvm construction.

What exactly are the pro's and con's of using lvm in this situaion? The documentation isn't very clear about this.

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answered 2014-04-10 15:58:37 -0500


The LVM code in nova does not currently support the thin LVM target (I believe that's what you want). However there is work on a subproject called Brick (currently in Cinder) that supports the new LVM thin target. The goal for Brick is to be a common library for other projects, nova being a candidate for that, however, this will probably happen in the K, L or another future release. It's unsure.

That being said, there is nothing stopping someone from modifying the current LVM implementation for ephemeral/root disk in nova to also support thin.

While the LVM implementation works as is (without thin) there are some limitations like not being able to live migrate a VM and probably other minor ones like the "need" to issue a dd on a LV after removal (this is valid for Icehouse and prior versions).

The only way to actually know for sure if LVM is ok for your needs is to continue testing your current deployment.

To me LVM feels left out (for root/ephemeral disks, not volumes), like someone added support for it in the past and now everyone is avoiding it while adding support for new features that work with qcow2 only.

As for the pros and cons, there are many discussions and nothing indicates that one solution is better than the other and there will probably never be a definite answer. It depends on your needs mostly and if you can get passed the limitations you'll encounter.

OpenStack seems to support qcow2 pretty well. LVM feels like a second class citizen in Openstack, at least to me. On the other hand I know large deployments that use LVM and for some reason (lack of information on my part probably) I can't find known virtualization providers that use qcow2 exclusively.

If you absolutely want thin provisioning then you should go with qcow2 for now.

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Thnx. This is good info. I found a nice article on the life of a openstack image. This gives good info on how the internals work (

atze.devries gravatar imageatze.devries ( 2014-04-11 06:44:09 -0500 )edit

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Asked: 2014-04-09 09:58:07 -0500

Seen: 1,856 times

Last updated: Apr 10 '14