From: | Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> |
---|---|
To: | alvherre(at)2ndquadrant(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: | 2020-04-01 01:18:04 |
Message-ID: | 20200401.101804.1527965430575324083.horikyota.ntt@gmail.com |
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Lists: | pgsql-hackers |
At Tue, 31 Mar 2020 14:18:36 -0300, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote in
> 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
> FindOldestXLogFileSegNo(void)
> {
> 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:
If we don't scan the wal files, for example (somewhat artificail), if
segments canoot be removed by a wrong setting of archive_command,
GetWalAvailability can return false "removed(lost)" state. If
max_slot_wal_keep_size is shrinked is changed then restarted, the
function can return false "normal" or "keeping" states.
By the way the oldest segment of initdb'ed cluster was (14x)th for
me. So I think we can treat segno == 1 as "uncertain" or "unknown"
state, but that state lasts until a checkpoint actually removes a
segment.
> XLogSegNo
> FindOldestXLogFileSegNo(void)
> {
> return XLogGetLastRemovedSegno() + 1;
> }
>
>
> The tests still pass with this coding.
Mmm. Yeah, that affects when under an abnormal condition.
regards.
--
Kyotaro Horiguchi
NTT Open Source Software Center
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