Re: Freezing without cleanup lock

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