Re: Multixacts wraparound monitoring

I understand correctly, that number of members cannot be more than 2^32 (also uses a 32-bit counter)?
I had 69640 files in main/pg_multixact/members/, 69640*32*2045 = 4557241600 members, this is normal?

Kind regards,
 
Vladimir Pavlov

-----Original Message-----
From: Alvaro Herrera [mailto:alvherre(at)2ndquadrant(dot)com]
Sent: Thursday, March 31, 2016 4:17 PM
To: Pavlov Vladimir
Cc: 'Adrian Klaver'; 'pgsql-general(at)postgresql(dot)org'
Subject: Re: [GENERAL] Multixacts wraparound monitoring

Pavlov, Vladimir wrote:
> Hello,
> If I get you right:
> Latest checkpoint's NextMultiXactId: 2075246000
> Latest checkpoint's oldestMultiXid: 2019511697
> Number of members files: 10820
> Size pg_multixact/members/ (bytes) (2.7Gb): 2887696384
> Pages in file: 32
> Members on page: 2045
> Number of members (32*2045*10820): 708060800
> Members per multixact (2075246000 - 2019511697)/708060800: 12,70421916
> Multixact size (bytes) (2887696384/708060800): 4,078316981 - It's a lot?

Yeah, 12.7 members per multixact on average is a lot, unless you have 12 processes concurrently locking the same tuples, all the time (although
that is possible). My guess is that this is related to subtransactions
(either explicit SAVEPOINTs in your SQL code, or EXCEPTION blocks in plpgsql functions).

--
Álvaro Herrera http://www.2ndQuadrant.com/
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