mgr/dashboard: improve rbd listing

[epuertat]

Adds TTL caching to ioctx and RBD images in order to speed up listing of many RBD images (> hundreds). Instantiating RBD images is one of the bottlenecks, so by reusing those, only when the TTL cache is cold this method takes longer to complete.

In order to improve the TTL caching, a ttl_spread has been added to the cache, so the items start to progressively expire from ttl*(1 - ttl_spread)/2 to ttl*(1 + ttl_spread)/2.

CAVEATS

• This assumes that ioctx and RBD image objects will be automatically deallocated by the Python ref-counting mechanism (as no context manager is used or explicit close is called).
• This still does not fix the underlying issue: since the number of RBD images may be a scaling factor for a Ceph cluster, the latency might sooner or later exceed the poll period.
• The only fix for that is to introduce pagination. However, that will break some Dashboard features, like the ability to sort RBD images by any property (size, number of objects, provisioned size, etc.), or the ability to filter those images by the same properties. A possibility, already applied in other endpoints, is to disable the periodic polling, while allowing the user to manually refresh the table.

Fixes: http://tracker.ceph.com/issues/39140
Signed-off-by: Ernesto Puerta epuertat@redhat.com

Checklist

• References tracker ticket
• Updates documentation if necessary
• Includes tests for new functionality or reproducer for bug
• Asynchronous expiration in the ttl cache.

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