Raising stop and warn limits
От | Noah Misch |
---|---|
Тема | Raising stop and warn limits |
Дата | |
Msg-id | 20200621083513.GA3074645@rfd.leadboat.com обсуждение исходный текст |
Ответы |
Re: Raising stop and warn limits
(Noah Misch <noah@leadboat.com>)
|
Список | pgsql-hackers |
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.GA1139206@rfd.leadboat.com and https://postgr.es/m/20200525070033.GA1591335@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
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