From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
Cc: | tarvip(at)gmail(dot)com, pgsql-bugs(at)postgresql(dot)org, Andres Freund <andres(at)2ndQuadrant(dot)com> |
Subject: | Re: BUG #7710: Xid epoch is not updated properly during checkpoint |
Date: | 2012-12-02 15:25:20 |
Message-ID: | 11810.1354461920@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> I've applied an absolutely minimal fix on this, which introduces no
> other changes that could cause unforeseen consequences.
This is not what we'd agreed to do, I thought.
Now that I've thought more about this bug, the existing coding is flat
out wrong, with or without correction of the epoch. As you yourself
just wrote in a comment, the checkpoint record logically belongs to the
"redo" point in the WAL stream, not to where it's physically located.
Having it carry a nextXid that belongs to the later point is simply
wrong. Having it carry different nextXids depending on wal_level is
even more wrong.
I can point right now to one misbehavior this causes: if you run a
point-in-time recovery with a stop point somewhere in the middle of the
checkpoint, you should end up with a nextXid corresponding to the stop
point. This hack in LogStandbySnapshot causes you to end up with a
much later nextXid, if you were running hot-standby.
> Others may wish to go further, overriding my patches, as they choose.
Okay, I will take the responsibility for changing this, but it needs to
change. This coding was ill-considered from the word go.
regards, tom lane
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