Improving latency and ordering of the backfilling workload

[Loic Dachary <loic@dachary.org>]

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Hi Sam,

Here is what could be done (in the context of http://tracker.ceph.com/issues/9566
), please let me know if that makes sense:

* ordering:

  * when dequeuing a pending local reservation, chose one that contains a PG that belongs to the busiest OSD (i.e. the OSD for which there are more PGs waiting for a local reservation than any other)
  * when sending a remote reservation request, set the priority to reflect the total number of pending PG (absolute workload) and the number local pending PG for the destination OSD (workload queued locally for the remote OSD)
  * on the receiving side, the priority of the remote reservation request makes sure the busiest OSD gets a remote reservation before the others

* reducing latency:
  
  * if there are N pending remote reservations, reject a remote reservation request instead of queuing it so that the local reservation can be used instead of waiting.

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre


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