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Re: [HACKERS] Incrementally Updated Backups and restartpoints

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, pgsql-docs(at)postgresql(dot)org
Subject: Re: [HACKERS] Incrementally Updated Backups and restartpoints
Date: 2010-03-26 13:21:42
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Lists: pgsql-docspgsql-hackers
On Thu, Mar 4, 2010 at 9:00 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> And, when we start an archive recovery from the backup from the standby,
>> we seem to reach a safe starting point before database has actually become
>> consistent. It's because backupStartLoc is zero. Isn't this an issue?
> This issue seems to still happen. So should this be fixed for 9.0?
> Or only writing a note in document is enough for 9.0? I'm leaning
> towards the latter.

I'm thinking of adding something like the following to the section
"25.6. Incrementally Updated Backups". Thought?

    The pg_control file must be backed up first.
    This avoids the problem that we might fail to restore a consistent
    database state because recovery starts from the later restart point
    than the start of the backup.

    When recovering from the incrementally updated backup, the server
    can begin accepting connections and complete the recovery successfully
    before the database has become consistent. To avoid these problems,
    you must check whether the database has been consistent by comparing
    the progress of the recovery with the backup ending WAL location
    before your users try to connect to the server and when archive
    recovery ends. So, in advance, the backup ending WAL location must
    be taken by calling the pg_last_xlog_replay_location function at the
    end of the backup. The progress of the recovery is also taken from
    the pg_last_xlog_replay_location function.


Fujii Masao
NTT Open Source Software Center

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