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doc/: added documentation for backfill_reservation

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Signed-off-by: Samuel Just <sam.just@inktank.com>
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Samuel Just 2012-09-07 09:22:10 -07:00
parent 796b9b6c2a
commit 3ebf52dfb5
3 changed files with 33 additions and 1 deletions
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3
doc/cluster-ops/pg-states.rst
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@ -49,6 +49,9 @@ map is ``active + clean``.
instead of inferring what contents need to be synchronized from the logs of
recent operations. *Backfill* is a special case of recovery.
*Wait-backfill*
The placement group is waiting in line to start backfill.
*Incomplete*
Ceph detects that a placement group is missing a necessary period of history
from its log. If you see this state, report a bug, and try to start any

27
doc/dev/osd_internals/backfill_reservation.rst Normal file
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@ -0,0 +1,27 @@
====================
Backfill Reservation
====================
When a new osd joins a cluster, all pgs containing it must eventually backfill
to it. If all of these backfills happen simultaneously, it would put excessive
load on the osd. osd_num_concurrent_backfills limits the number of outgoing or
incoming backfills on a single node.
Each OSDService now has two AsyncReserver instances: one for backfills going
from the osd (local_reserver) and one for backfills going to the osd
(remote_reserver). An AsyncReserver (common/AsyncReserver.h) manages a queue
of waiting items and a set of current reservation holders. When a slot frees
up, the AsyncReserver queues the Context* associated with the next item in the
finisher provided to the constructor.
For a primary to initiate a backfill, it must first obtain a reservation from
its own local_reserver. Then, it must obtain a reservation from the backfill
target's remote_reserver via a MBackfillReserve message. This process is
managed by substates of Active and ReplicaActive (see the substates of Active
in PG.h). The reservations are dropped either on the Backfilled event, which
is sent on the primary before calling recovery_complete and on the replica on
receipt of the BackfillComplete progress message), or upon leaving Active or
ReplicaActive.
It's important that we always grab the local reservation before the remote
reservation in order to prevent a circular dependency.

4
doc/dev/placement-group.rst
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@ -129,6 +129,9 @@ User-visible PG States
the PG are scanned and synchronized, instead of inferring what
needs to be transferred from the PG logs of recent operations
*backfill-wait*
the PG is waiting in line to start backfill
*incomplete*
a pg is missing a necessary period of history from its
log. If you see this state, report a bug, and try to start any
@ -141,4 +144,3 @@ User-visible PG States
*remapped*
the PG is temporarily mapped to a different set of OSDs from what
CRUSH specified