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Dear Joe,

Thanks for your kind reply, your informations are helpful. I'm reading 
the imagecache.py[1] sourcecode in order to really understand what it'll 
happen in case of a shared filesystem.

I understand the SHA1 hash mechanism and the backing file check but I'm 
not sure how it will manage the case of shared FS.

The main function seems to be :
- backing_file = libvirt_utils.get_disk_backing_file(disk_path)

But does libvirt_utils.get_disk_backing_file federates all the compute 
nodes informations ?! If no it may delete the other nodes images ?

Hope it's not too redundant,
Kind regards

[1] 
https://github.com/openstack/nova/blob/master/nova/virt/libvirt/imagecache.py

Le 28/04/2015 16:18, Joe Topjian a écrit :
> Hello,
>
>     I've got a similar question about cache-manager and the presence
>     of a shared filesystem for instances images.
>     I'm currently reading the source code in order to find out how
>     this is managed but before I would be curious how you achieve this
>     on production servers.
>
>     For example images not used by compute node A will probably be
>     cleaned on the shared FS despite the fact that compute B use it,
>     that's the main problem.
>
>
> This used to be a problem, but AFAIK it should not happen any more. If 
> you're noticing it happening, please raise a flag.
>
>     How do you handle _base guys ?
>
>
> We configure Nova to not have instances rely on _base files. We found 
> it to be too dangerous of a single point of failure. For example, we 
> ran into the scenario you described a few years ago before it was 
> fixed. Bugs are one thing, but there are a lot of other ways a _base 
> file can become corrupt or removed. Even if those scenarios are rare, 
> the results are damaging enough for us to totally forgo reliance of 
> _base files.
>
> Padraig Brady has an awesome article that details the many ways you 
> can configure _base and instance files:
>
> http://www.pixelbeat.org/docs/openstack_libvirt_images/
>
> I'm looping -operators into this thread for input on further ways to 
> handle _base. You might also be able to find some other methods by 
> searching the -operators mailing list archive.
>
> Thanks,
> Joe
>

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