[Openstack-operators] [nova] Is verification of images in the image cache necessary?

John Garbutt john at johngarbutt.com
Tue May 24 10:06:00 UTC 2016

On 24 May 2016 at 10:16, Matthew Booth <mbooth at redhat.com> wrote:
> During its periodic task, ImageCacheManager does a checksum of every image
> in the cache. It verifies this checksum against a previously stored value,
> or creates that value if it doesn't already exist.[1] Based on this
> information it generates a log message if the image is corrupt, but
> otherwise takes no action. Going by git, this has been the case since 2012.
> The commit which added it was associated with 'blueprint
> nova-image-cache-management phase 1'. I can't find this blueprint, but I did
> find this page: https://wiki.openstack.org/wiki/Nova-image-cache-management
> . This talks about 'detecting images which are corrupt'. It doesn't explain
> why we would want to do that, though. It also doesn't seem to have been
> followed through in the last 4 years, suggesting that nobody's really that
> bothered.
> I understand that corruption of bits on disks is a thing, but it's a thing
> for more than just the image cache. I feel that this is a problem much
> better solved at other layers, prime candidates being the block and
> filesystem layers. There are existing robust solutions to bitrot at both of
> these layers which would cover all aspects of data corruption, not just this
> randomly selected slice.


That might mean improved docs on the need to configure such a thing.

> As it stands, I think this code is regularly running a pretty expensive task
> looking for something which will very rarely happen, only to generate a log
> message which nobody is looking for. And it could be solved better in other
> ways. Would anybody be sad if I deleted it?

For completeness, we need to deprecate it using the usual cycles:

I like the idea of checking the md5 matches before each boot, as it
mirrors the check we do after downloading from glance. Its possible
thats very unlikely to spot anything that shouldn't already be worried
about by something else. It may just be my love of symmetry that makes
me like that idea?


> [1] Incidentally, there also seems to be a bug in this implementation, in
> that it doesn't hold the lock on the image itself at any point during the
> hashing process, meaning that it cannot guarantee that the image has
> finished downloading yet.
> --
> Matthew Booth
> Red Hat Engineering, Virtualisation Team
> Phone: +442070094448 (UK)
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