From: Sage Weil <sage@redhat.com>
Date: Tue, 9 Apr 2019 22:12:37 +0000 (-0500)
Subject: mgr/DaemonServer: prevent pgp_num reductions from outpacing pg_num merges
X-Git-Tag: v14.2.1~23^2
X-Git-Url: http://git-server-git.apps.pok.os.sepia.ceph.com/?a=commitdiff_plain;h=refs%2Fpull%2F27547%2Fhead;p=ceph.git

mgr/DaemonServer: prevent pgp_num reductions from outpacing pg_num merges

If we are merging lots of pgs down to a much smaller number of pgs, and
the pgs are able to move quickly (faster than the merges happen), we can
end up with too many pgs on a small number of osds, triggering the max
pgs per osd limits.

Avoid this by preventing the pgp_num reductions from getting too far
out in front of the merges themselves.  Basically, cap the delta between
pgp_num and pg_num to the max_misplaced ratio.  We are already limiting
the movement caused by pgp_num by max_misplaced; this effectively just
makes sure that the actual merging (and pg_num reductions) are keeping
up.

Fixes: http://tracker.ceph.com/issues/38786
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 76503a1438fa1f166d2c230c73ca8d7b67e6468d)
---

diff --git a/src/mgr/DaemonServer.cc b/src/mgr/DaemonServer.cc
index cf1d79bf5ef9..38143ecc227f 100644
--- a/src/mgr/DaemonServer.cc
+++ b/src/mgr/DaemonServer.cc
@@ -2601,6 +2601,22 @@ void DaemonServer::adjust_pgs()
 	      }
 	      dout(20) << " room " << room << " estmax " << estmax
 		       << " delta " << delta << " next " << next << dendl;
+	      if (p.get_pgp_num_target() == p.get_pg_num_target()) {
+		// since pgp_num is tracking pg_num, ceph is handling
+		// pgp_num.  so, be responsible: don't let pgp_num get
+		// too far out ahead of merges (if we are merging).
+		// this avoids moving lots of unmerged pgs onto a
+		// small number of OSDs where we might blow out the
+		// per-osd pg max.
+		unsigned max_outpace_merges =
+		  std::max<unsigned>(8, p.get_pg_num() * max_misplaced);
+		if (next + max_outpace_merges < p.get_pg_num()) {
+		  next = p.get_pg_num() - max_outpace_merges;
+		  dout(10) << "  using next " << next
+			   << " to avoid outpacing merges (max_outpace_merges "
+			   << max_outpace_merges << ")" << dendl;
+		}
+	      }
 	    }
 	    dout(10) << "pool " << i.first
 		     << " pgp_num_target " << p.get_pgp_num_target()