Hmm. Not sure how widespread installations with multiple Ceph backends are 
where the
Cinder hosts have access to only one of the backends (which is what you assume, 
But, yes, if the volume type names are also the same (is that also needed for 
this to be a
problem?), this will be an issue ...

So, how about providing the information the scheduler does not have by 
introducing an
additional tag to identify ‘equivalent’ backends, similar to the way some 
people already
use the ‘host’ option?


On 08 Jan 2015, at 15:11, Duncan Thomas 
<<>> wrote:

The problem is that the scheduler doesn't currently have enough info to know 
which backends are 'equivalent' and which aren't. e.g. If you have 2 ceph 
clusters as cinder backends, they are indistinguishable from each other.

On 8 January 2015 at 12:14, Arne Wiebalck 
<<>> wrote:

The fact that volume requests (in particular deletions) are coupled with 
certain Cinder hosts is not ideal from an operational perspective:
if the node has meanwhile disappeared, e.g. retired, the deletion gets stuck 
and can only be unblocked by changing the database. Some
people apparently use the ‘host’ option in cinder.conf to make the hosts 
indistinguishable, but this creates problems in other places.

From what I see, even for backends that would support it (such as Ceph), Cinder 
currently does not provide means to ensure that any of
the hosts capable of performing a volume operation would be assigned the 
request in case the original/desired one is no more available,

If that is correct, how about changing the scheduling of delete operation to 
use the same logic as create operations, that is pick any of the
available hosts, rather than the one which created a volume in the first place 
(for backends where that is possible, of course)?


Arne Wiebalck
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Duncan Thomas
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