Improving latency and ordering of the backfilling workload

[Loic Dachary]

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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