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Hi Jordan,

As Duncan pointed out there may be issues if you have multiple backends
and indistinguishable nodes (which you could  probably avoid by separating
the hosts per backend and use different “host” flags for each set).

But also if you have only one backend: the “host" flag will enter the ‘services'
table and render the host column more or less useless. I imagine this has an
impact on things using the services table, such as “cinder-manage” (how
does your “cinder-manage service list” output look like? :-), and it may make it
harder to tell if the individual services are doing OK, or to control them.

I haven’t run Cinder with identical “host” flags in production, but I imagine
there may be other areas which are not happy about indistinguishable hosts.

Arne


On 08 Jan 2015, at 16:50, Jordan Pittier <jordan.pittier at scality.com<mailto:jordan.pittier at scality.com>> wrote:

Hi,
>Some people apparently use the ‘host’ option in cinder.conf to make the hosts indistinguishable, but this creates problems in other places.
I use shared storage mounted on several cinder-volume nodes, with "host" flag set the same everywhere. Never ran into problems so far. Could you elaborate on "this creates problems in other places" please ?

Thanks !
Jordan

On Thu, Jan 8, 2015 at 3:40 PM, Arne Wiebalck <Arne.Wiebalck at cern.ch<mailto:Arne.Wiebalck at cern.ch>> wrote:
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, right?)
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?

Thanks!
 Arne


On 08 Jan 2015, at 15:11, Duncan Thomas <duncan.thomas at gmail.com<mailto:duncan.thomas at gmail.com>> 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 <Arne.Wiebalck at cern.ch<mailto:Arne.Wiebalck at cern.ch>> wrote:
Hi,

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,
right?

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)?

Thanks!
 Arne

—
Arne Wiebalck
CERN IT
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