Re: Is rbd_discard enough to wipe an RBD image?

[Wido den Hollander <wido@42on.com>]

On 12/21/2015 04:50 PM, Josh Durgin wrote:
> On 12/21/2015 07:09 AM, Jason Dillaman wrote:
>> You will have to ensure that your writes are properly aligned with the
>> object size (or object set if fancy striping is used on the RBD
>> volume).  In that case, the discard is translated to remove operations
>> on each individual backing object.  The only time zeros are written to
>> disk is if you specify an offset somewhere in the middle of an object
>> (i.e. the whole object cannot be deleted nor can it be truncated) --
>> this is the partial discard case controlled by that configuration param.
>>
> 
> I'm curious what's using the virVolWipe stuff - it can't guarantee it's
> actually wiping the data in many common configurations, not just with
> ceph but with any kind of disk, since libvirt is usually not consuming
> raw disks, and with modern flash and smr drives even that is not enough.
> There's a recent patch improving the docs on this [1].
> 
> If the goal is just to make the data inaccessible to the libvirt user,
> removing the image is just as good.
> 
> That said, with rbd there's not much cost to zeroing the image with
> object map enabled - it's effectively just doing the data removal step
> of 'rbd rm' early.
> 

I was looking at the features the RBD storage pool driver is missing in
libvirt and it is:

- Build from Volume. That's RBD cloning
- Uploading and Downloading Volume
- Wiping Volume

The thing about wiping in libvirt is that the volume still exists
afterwards, it is just empty.

My discard code now works, but I wanted to verify. If I understand Jason
correctly it would be a matter of figuring out the 'order' of a image
and call rbd_discard in a loop until you reach the end of the image.

I just want libvirt to be as feature complete as possible when it comes
to RBD.

Wido

> Josh
> 
> [1] http://comments.gmane.org/gmane.comp.emulators.libvirt/122235
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-- 
Wido den Hollander
42on B.V.
Ceph trainer and consultant

Phone: +31 (0)20 700 9902
Skype: contact42on