postgres.git / summary / log / commit / refs

commit    b4d4ce1d50bbdf82cd2e2c1c7172b936df01c01d
Author:   Robert Haas <rhaas@postgresql.org>
Date:     Mon May 11 16:07:13 2015 +0000
First released in: 9.5.0, 9.3.7, 9.4.2

    Increase threshold for multixact member emergency autovac to 50%.
    
    Analysis by Noah Misch shows that the 25% threshold set by commit
    53bb309d2d5a9432d2602c93ed18e58bd2924e15 is lower than any other,
    similar autovac threshold.  While we don't know exactly what value
    will be optimal for all users, it is better to err a little on the
    high side than on the low side.  A higher value increases the risk
    that users might exhaust the available space and start seeing errors
    before autovacuum can clean things up sufficiently, but a user who
    hits that problem can compensate for it by reducing
    autovacuum_multixact_freeze_max_age to a value dependent on their
    average multixact size.  On the flip side, if the emergency cap
    imposed by that patch kicks in too early, the user will experience
    excessive wraparound scanning and will be unable to mitigate that
    problem by configuration.  The new value will hopefully reduce the
    risk of such bad experiences while still providing enough headroom
    to avoid multixact member exhaustion for most users.
    
    Along the way, adjust the documentation to reflect the effects of
    commit 04e6d3b877e060d8445eb653b7ea26b1ee5cec6b, which taught
    autovacuum to run for multixact wraparound even when autovacuum
    is configured off.


 doc/src/sgml/maintenance.sgml          | 7 ++++---
 src/backend/access/transam/multixact.c | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index 211ad03d954..e34426ff18f 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -658,11 +658,12 @@ HINT:  Stop the postmaster and vacuum that database in single-user mode.
     <para>
      As a safety device, a whole-table vacuum scan will occur for any table
      whose multixact-age is greater than
-     <xref linkend="guc-autovacuum-multixact-freeze-max-age">.
-     This will occur even if autovacuum is nominally disabled.  Whole-table
+     <xref linkend="guc-autovacuum-multixact-freeze-max-age">.  Whole-table
      vacuum scans will also occur progressively for all tables, starting with
      those that have the oldest multixact-age, if the amount of used member
-     storage space exceeds the amount 25% of the addressible storage space.
+     storage space exceeds the amount 50% of the addressible storage space.
+     Both of these kinds of whole-table scans will occur even if autovacuum is
+     nominally disabled.
     </para>
    </sect3>
   </sect2>
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index fc0e4e74004..d92cbccaf8a 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -169,7 +169,7 @@
 	 ((xid) % MULTIXACT_MEMBERS_PER_MEMBERGROUP) * sizeof(TransactionId))
 
 /* Multixact members wraparound thresholds. */
-#define MULTIXACT_MEMBER_SAFE_THRESHOLD		(MaxMultiXactOffset / 4)
+#define MULTIXACT_MEMBER_SAFE_THRESHOLD		(MaxMultiXactOffset / 2)
 #define MULTIXACT_MEMBER_DANGER_THRESHOLD	\
 	(MaxMultiXactOffset - MaxMultiXactOffset / 4)
 


[parent: 2200713aa8bb]