Re: [HACKERS] Restricting maximum keep segments by repslots

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Cc: jgdr(at)dalibo(dot)com, andres(at)anarazel(dot)de, michael(at)paquier(dot)xyz, sawada(dot)mshk(at)gmail(dot)com, peter(dot)eisentraut(at)2ndquadrant(dot)com, pgsql-hackers(at)lists(dot)postgresql(dot)org, thomas(dot)munro(at)enterprisedb(dot)com, sk(at)zsrv(dot)org, michael(dot)paquier(at)gmail(dot)com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
Date: 2020-03-31 17:18:36
Message-ID: 20200331171836.GA7973@alvherre.pgsql
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On 2020-Mar-31, Alvaro Herrera wrote:

> I'm not sure if I explained my proposal clearly. What if
> XLogGetLastRemovedSegno returning zero means that every segment is
> valid? We don't need to scan pg_xlog at all.

I mean this:

XLogSegNo segno = XLogGetLastRemovedSegno();

/* this is the only special case we need to care about */
if (segno == 0)
return some-value;

return segno + 1;

... and that point one can further note that a freshly initdb'd system
(no file has been removed) has "1" as the first file. So when segno is
0, you can return 1 and all should be well. That means you can reduce
the function to this:

return XLogGetLastRemovedSegno() + 1;

The tests still pass with this coding.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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