[libvirt] [PATCH 1/2] rbd: Implement wiping RBD volumes using rbd_discard()

[Wido den Hollander]

On 23-12-15 14:05, Daniel P. Berrange wrote:
> On Wed, Dec 23, 2015 at 01:50:52PM +0100, Wido den Hollander wrote:
>>
>>
>> On 23-12-15 10:45, Daniel P. Berrange wrote:
>>> On Wed, Dec 23, 2015 at 10:29:04AM +0100, Wido den Hollander wrote:
>>>> This allows user to use the volume wiping functionality of the libvirt
>>>> storage driver.
>>>>
>>>> All data from the volume will be wiped by calling rbd_discard() in chunks of the
>>>> underlying object and stripe size inside Ceph.
>>>>
>>>> This will ensure all data is zeroed and leaves the user with a completely empty volume
>>>> ready for use again.
>>>
>>> Based on the name 'rbd_discard' it sounds like this is going to call
>>> TRIM/DISCARD on the underlying storage too ? If so, then I don't think
>>> that this is an appropriate approach for this API. The virStorageVolWipe
>>> API should clear the data, *without* having any effect on the storage of
>>> the API - ie we don't want to discard underling storage blocks as a
>>> side effect
>>>
>>
>> Afaik it zeroes/trims all the RBD objects on the Ceph cluster, but it
>> doesn't TRIM the lower SSD on it's turn.
>>
>> So it will send these calls to Ceph/RBD and it will zero all the data of
>> that specific volume. A rather simple way to get rid of the data in a
>> volume.
> 
> That's not what I see that API impl of rbd_discard() doing. It
> looks very much like it is discarding extents from the RBD volume,
> at least if the discarded region is large enough. Only if the discarded
> region is small, does it merely zero the data.
> 

Let me verify this with the Ceph people.

> So I really don't think this is a suitable API for use with the
> virStorageVolWipe() API, whose *only* effect should be to overwrite
> the data, not have any side effect on volume extent allocation
> 

What do you suggest? use rbd_write() in a loop and overwrite the whole
volume?

The problem with this option is that the RBD volume will then grow to
it's maximum size on the underlying storage.

With rbd_discard() being called on the exact object size all those
objects will be trimmed, effectively wiping the volume's data. So I
figured it was OK.

I just want to make the RBD storage driver as feature complete as
possible :)

Wido

> 
> Regards,
> Daniel
>