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Re: Streaming replication and pg_xlogfile_name()

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Erik Rijkers <er(at)xs4all(dot)nl>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication and pg_xlogfile_name()
Date: 2010-03-02 13:52:16
Message-ID: 3f0b79eb1003020552q3135bb0ey616dae796f00ab87@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Mar 2, 2010 at 8:54 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> This still suffers from ambiguity around a shutdown checkpoint that
> changes the TLI. On the page the shutdown checkpoint is on, what is the
> TLI in the page header? The TLI before the checkpoint record, I presume.
> Now consider a record on the same page after the checkpoint record. It's
> on the new timeline, but pg_last_xlog_replay_location() will return the
> old TLI, because that's on the page header.

Oh, I see. You are right.

> It's not clear what it should return, a TLI corresponding the filename
> of the WAL segment the record was replayed from, so that you can use
> pg_xlogfile_name() to find out the filename of the WAL segment being
> replayed, or the accurate TLI of the record being replayed. I'm leaning
> towards the latter, it feels more correct and accurate, but you could
> argue for the former too. In any case, it needs to be well-defined.

I agree with you that the latter is more correct and accurate. The simple
fix is updating the lastPageTLI with the CheckPoint->ThisTimeLineID when
replaying the shutdown checkpoint record. Though we might need to use new
variable to keep the last applied timeline instead of the lastPageTLI.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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