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pg_receivexlog and sync rep Re: Updated version of pg_receivexlog

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Magnus Hagander <magnus(at)hagander(dot)net>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Jaime Casanova <jaime(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: pg_receivexlog and sync rep Re: Updated version of pg_receivexlog
Date: 2012-02-08 18:39:41
Message-ID: CAHGQGwH3ojxHOCrEfF09URojA3o4y+s5s6=fi3yqa53t-fx08Q@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Oct 25, 2011 at 7:37 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Mon, Oct 24, 2011 at 14:40, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>> On Mon, Oct 24, 2011 at 13:46, Heikki Linnakangas
>> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> How does this interact with synchronous replication? If a base backup that
>>> streams WAL is in progress, and you have synchronous_standby_names set to
>>> '*', I believe the in-progress backup will count as a standby for that
>>> purpose. That might give a false sense of security.
>>
>> Ah yes. Did not think of that. Yes, it will have this problem.
>
> Actually, thinking more, per other mail, it won't. Because it will
> never report that the data is synced to disk, so it will not be
> considered for sync standby.

Now, new replication mode (synchronous_commit = write) is supported.
In this mode, the in-progress backup will be considered as sync
standby because its periodic status report includes the valid write position.
We should change the report so that it includes only invalid positions.
Patch attached.

While I agree that the backup should not behave as sync standby, ISTM
that pg_receivexlog should, which is very useful. If pg_receivexlog does
so, we can write WAL synchronously in both local and remote, which
would increase the durability of the system. Thus, to allow pg_receivexlog
to behave as sync standby, we should change it so that its status report
includes the write and flush positions?

Regards,

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

Attachment: invalid_write_pos_v1.patch
Description: application/octet-stream (613 bytes)

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