On 10/21/15 3:14 PM, Alvaro Herrera wrote:
Jim Nasby wrote:
While warning a client that just did a Slony-based version upgrade to make
sure to freeze the new database, it occurred to me that it should be safe to
freeze without the cleanup lock. This is interesting because it would allow
a scan_all vacuum to do it's job without blocking on the cleanup lock.

Does anyone have a feel for whether scan_all vacuums blocking on the cleanup
lock is an actual problem?

Yeah, I remember we discussed this and some other possible improvements
related to freezing.  I think other ideas proposed were that (1) during
an emergency (uncancellable) autovacuum run, we process only the tables
that are past the age limit, and (2) we remove the cost-based sleep so
that it finishes as quickly as possible.  (Yours is (3) only freeze and
not do any actual pruning -- did I get that right?)

That would be the minimal-impact version, yes. But I suspect if we went through the trouble to do that, it would be just as easy to attempt the freeze regardless of what scan_all is set to.

What I wish I knew is whether this problem was worth worrying about or not. Hopefully the extra logging in 9.5 will shed some light at some point...
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com

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