[libvirt] libvirt can not get right stats of a rbd pool

[Wido den Hollander]

On 10/29/2012 03:33 PM, Daniel P. Berrange wrote:
> On Fri, Oct 26, 2012 at 11:04:05AM +0800, yue wrote:
>> Allocation exceed Capacity ,but Available  is  not 0.
>>
>> #virsh pool-info 2361a6d4-0edc-3534-87ae-e7ee09199921
>> Name:           2361a6d4-0edc-3534-87ae-e7ee09199921
>> UUID:           2361a6d4-0edc-3534-87ae-e7ee09199921
>> State:          running
>> Persistent:     yes
>> Autostart:      no
>> Capacity:       285.57 GiB
>> Allocation:     489.89 GiB
>> Available:      230.59 GiB
>
> Hmm, these values do look a little bit suspect, but I don't know
> enough about RBD to suggest what might be going wrong. I'm copying
> Wido who wrote this code originally & thus might have an idea.
>

I think I know where this is coming from, a little background about RBD.

RBD is a disk device striped over 4MB RADOS objects inside a Ceph 
cluster. RBD devices are sparse, which means that (RADOS) objects get 
created whenever a write comes.

When a read comes for a non-existing object 4MB of nothing is returned.

However, when you do: $ rbd info disk1

You will get that the object COULD be 100GB, but that doesn't mean it 
actually occupies 100GB of disk space.

The problem is that you can't (at this point) find out how much space a 
RBD device actually occupies. Yes, it can be done, but that should not 
be done in the libvirt driver and it is pretty heavy for the Ceph cluster.

Also, a RBD device of 100GB could take up 300GB of space when your 
replication is set to 3x.

What you are seeing there is that you over provisioned your Ceph cluster 
by creating images which exceed 285GB in total.

Wido

> Daniel
>