Re: Raising stop and warn limits
On Sun, Jun 21, 2020 at 01:35:13AM -0700, Noah Misch wrote: > In brief, I'm proposing to raise xidWrapLimit-xidStopLimit to 3M and > xidWrapLimit-xidWarnLimit to 40M. Likewise for mxact counterparts. Here's the patch for it. > 1. VACUUM, to advance a limit, may assign IDs subject to one of the limits. >VACUUM formerly consumed XIDs, not mxacts. It now consumes mxacts, not >XIDs. Correction: a lazy_truncate_heap() at wal_level!=minimal does assign an XID, so XID consumption is impossible with "VACUUM (TRUNCATE false)" but possible otherwise. "VACUUM (ANALYZE)", which a DBA might do by reflex, also assigns XIDs. (These corrections do not affect $SUBJECT.) Author: Noah Misch Commit: Noah Misch Change XID and mxact limits to warn at 40M and stop at 3M. We have edge-case bugs when assigning values in the last few dozen pages before the wrap limit. We may introduce similar bugs in the future. At default BLCKSZ, this makes such bugs unreachable outside of single-user mode. Also, when VACUUM began to consume mxacts, multiStopLimit did not change to compensate. pg_upgrade may fail on a cluster that was already printing "must be vacuumed" warnings. Follow the warning's instructions to clear the warning, then run pg_upgrade again. One can still, peacefully consume 98% of XIDs or mxacts, so DBAs need not change routine VACUUM settings. Reviewed by FIXME. Discussion: https://postgr.es/m/20200621083513.ga3074...@rfd.leadboat.com diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml index 612e4cb..a28ea56 100644 --- a/doc/src/sgml/maintenance.sgml +++ b/doc/src/sgml/maintenance.sgml @@ -608,10 +608,10 @@ SELECT datname, age(datfrozenxid) FROM pg_database; If for some reason autovacuum fails to clear old XIDs from a table, the system will begin to emit warning messages like this when the database's -oldest XIDs reach eleven million transactions from the wraparound point: +oldest XIDs reach forty million transactions from the wraparound point: -WARNING: database "mydb" must be vacuumed within 10985967 transactions +WARNING: database "mydb" must be vacuumed within 39985967 transactions HINT: To avoid a database shutdown, execute a database-wide VACUUM in that database. @@ -621,7 +621,7 @@ HINT: To avoid a database shutdown, execute a database-wide VACUUM in that data be able to advance the database's datfrozenxid.) If these warnings are ignored, the system will shut down and refuse to start any new -transactions once there are fewer than 1 million transactions left +transactions once there are fewer than three million transactions left until wraparound: @@ -629,7 +629,7 @@ ERROR: database is not accepting commands to avoid wraparound data loss in data HINT: Stop the postmaster and vacuum that database in single-user mode. -The 1-million-transaction safety margin exists to let the +The three-million-transaction safety margin exists to let the administrator recover without data loss, by manually executing the required VACUUM commands. However, since the system will not execute commands once it has gone into the safety shutdown mode, diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index ce84dac..475f5ed 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -2217,28 +2217,24 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid, multiWrapLimit += FirstMultiXactId; /* -* We'll refuse to continue assigning MultiXactIds once we get within 100 -* multi of data loss. -* -* Note: This differs from the magic number used in -* SetTransactionIdLimit() since vacuum itself will never generate new -* multis. XXX actually it does, if it needs to freeze old multis. +* We'll refuse to continue assigning MultiXactIds once we get within 3M +* multi of data loss. See SetTransactionIdLimit. */ - multiStopLimit = multiWrapLimit - 100; + multiStopLimit = multiWrapLimit - 300; if (multiStopLimit < FirstMultiXactId) multiStopLimit -= FirstMultiXactId; /* -* We'll start complaining loudly when we get within 10M multis of the -* stop point. This is kind of arbitrary, but if you let your gas gauge -* get down to 1% of full, would you be looking for the next gas station? -* We need to be fairly liberal about this number because there are lots -* of scenarios where most transactions are done by automatic clients that -* won't pay attention to warnings. (No, we're not gonna make this +* We'll start complaining loudly when we get within 40M multis of data +* loss. This is kind of arbitrary, but if you let your
Raising stop and warn limits
In brief, I'm proposing to raise xidWrapLimit-xidStopLimit to 3M and xidWrapLimit-xidWarnLimit to 40M. Likewise for mxact counterparts. PostgreSQL has three "stop limit" values beyond which only single-user mode will assign new values of a certain counter: - xidStopLimit protects pg_xact, pg_commit_ts, pg_subtrans, and pg_serial. SetTransactionIdLimit() withholds a million XIDs, and warnings start ten million before that. - multiStopLimit protects pg_multixact/offsets. SetMultiXactIdLimit() withholds 100 mxacts, and warnings start at ten million. - offsetStopLimit protects pg_multixact/members. SetOffsetVacuumLimit() withholds [1,2) SLRU segments thereof (50k-100k member XIDs). No warning phase for this one. Reasons to like a larger stop limit: 1. VACUUM, to advance a limit, may assign IDs subject to one of the limits. VACUUM formerly consumed XIDs, not mxacts. It now consumes mxacts, not XIDs. I think a DBA can suppress VACUUM's mxact consumption by stopping all transactions older than vacuum_freeze_min_age, including prepared transactions. 2. We currently have edge-case bugs when assigning values in the last few dozen pages before the wrap limit (https://postgr.es/m/20190214072623.ga1139...@rfd.leadboat.com and https://postgr.es/m/20200525070033.ga1591...@rfd.leadboat.com). A higher stop limit could make this class of bug unreachable outside of single-user mode. That's valuable against undiscovered bugs of this class. 3. Any bug in stop limit enforcement is less likely to have user impact. For a live example, see the XXX comment that https://postgr.es/m/attachment/111084/slru-truncate-modulo-v3.patch adds to CheckPointPredicate(). Raising a stop limit prompts an examination of warn limits, which represent the time separating the initial torrent of warnings from the service outage. The current limits appeared in 2005; transaction rates have grown, while human reaction times have not. I like warnings starting when an SLRU is 98% consumed (40M XIDs or mxacts remaining). That doesn't change things enough to make folks reconfigure VACUUM, and it buys back some of the grace period DBAs had in 2005. I considered 95-97%, but the max_val of autovacuum_freeze_max_age would then start the warnings before the autovacuum. While that wouldn't rule out a value lower than 98%, 98% felt fine anyhow. For the new stop limits, I propose allowing 99.85% SLRU fullness (stop with 3M XIDs or mxacts remaining). If stopping this early will bother users, an alternative is 3M for XIDs and 0.2M for others. Either way leaves at least two completely-idle segments for each SLRU, which I expect to mitigate present and future edge-case bugs. Changing this does threaten clusters that experience pg_upgrade when close to a limit. pg_upgrade can fail or, worse, yield a cluster that spews warnings shortly after the upgrade. I could implement countermeasures, but they would take effect only when one upgrades a cluster having a 98%-full SLRU. I propose not to change pg_upgrade; some sites may find cause to do a whole-cluster VACUUM before pg_upgrade. Do you agree or disagree with that choice? I am attaching a patch (not for commit) that demonstrates the pg_upgrade behavior that nearly-full-SLRU clusters would see. Thanks, nm diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c index f3da40a..c43ebbf 100644 --- a/src/backend/access/transam/clog.c +++ b/src/backend/access/transam/clog.c @@ -795,6 +795,10 @@ TrimCLOG(void) int slotno; char *byteptr; + /* hack for pg_resetwal moving us to the middle of a page */ + if (!SimpleLruDoesPhysicalPageExist(XactCtl, pageno)) + SimpleLruZeroPage(XactCtl, pageno); + slotno = SimpleLruReadPage(XactCtl, pageno, false, xid); byteptr = XactCtl->shared->page_buffer[slotno] + byteno; diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index ce84dac..ed47ce0 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -2042,6 +2042,10 @@ TrimMultiXact(void) int slotno; MultiXactOffset *offptr; + /* hack for pg_resetwal moving us to the middle of a page */ + if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno)) + SimpleLruZeroPage(MultiXactOffsetCtl, pageno); + slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact); offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno]; offptr += entryno; diff --git a/src/bin/pg_resetwal/pg_resetwal.c b/src/bin/pg_resetwal/pg_resetwal.c index 2334418..454ff8b 100644 --- a/src/bin/pg_resetwal/pg_resetwal.c +++ b/src/bin/pg_resetwal/pg_resetwal.c @@ -434,6
