PG: don't write out pg map epoch every handle_activate_map

We don't actually need to write out the pg map epoch on every
activate_map as long as:
a) the osd does not trim past the oldest pg map persisted
b) the pg does update the persisted map epoch from time
to time.

To that end, we now keep a reference to the last map persisted.
The OSD already does not trim past the oldest live OSDMapRef.
Second, handle_activate_map will trim if the difference between
the current map and the last_persisted_map is large enough.

Fixes: #4731
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 2c5a9f0e178843e7ed514708bab137def840ab89)

Conflicts:

	src/common/config_opts.h
	src/osd/PG.cc
	- last_persisted_osdmap_ref gets set in the non-static
	  PG::write_info

Conflicts:

	src/osd/PG.cc

1 files changed, 4 insertions, 0 deletions

diff --git a/src/common/config_opts.h b/src/common/config_opts.h
index 5e87b2f1782..e23c8affb0f 100644
--- a/src/common/config_opts.h
+++ b/src/common/config_opts.h
@@ -435,6 +435,10 @@ OPTION(osd_use_stale_snap, OPT_BOOL, false)
 OPTION(osd_rollback_to_cluster_snap, OPT_STR, "")
 OPTION(osd_default_notify_timeout, OPT_U32, 30) // default notify timeout in seconds
 OPTION(osd_kill_backfill_at, OPT_INT, 0)
+
+// Bounds how infrequently a new map epoch will be persisted for a pg
+OPTION(osd_pg_epoch_persisted_max_stale, OPT_U32, 200)
+
 OPTION(osd_min_pg_log_entries, OPT_U32, 3000)  // number of entries to keep in the pg log when trimming it
 OPTION(osd_max_pg_log_entries, OPT_U32, 10000) // max entries, say when degraded, before we trim
 OPTION(osd_op_complaint_time, OPT_FLOAT, 30) // how many seconds old makes an op complaint-worthy