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Re: Streaming Replication: Checkpoint_segment and wal_keep_segments on standby

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, "Sander, Ingo (NSN - DE/Munich)" <ingo(dot)sander(at)nsn(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Streaming Replication: Checkpoint_segment and wal_keep_segments on standby
Date: 2010-05-31 10:17:03
Message-ID: AANLkTimpURYMtgdSlRrDO9MMbRDmPVMnQe522hBPdEB_@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, May 31, 2010 at 6:37 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> The central question is whether checkpoint_segments should trigger
> restartpoints or not. When PITR and restartpoints were introduced, the
> answer was "no", on the grounds that when you're doing recovery you're
> presumably replaying the logs much faster than they were generated, and you
> don't want to slow down the recovery by checkpointing too often.

Right.

> Now that we have bgwriter active during recovery, and streaming replication
> which retains the streamed WALs so that we now risk running out of disk
> space with long checkpoint_timeout, it's time to reconsider that.
>
> I think we have three options:
>
> 1) Leave it as it is, checkpoint_segments doesn't do anything during
> recovery/standby mode
>
> 2) Change it so that checkpoint_segments does take effect during
> recover/standby
>
> 3) Change it so that checkpoint_segments takes effect during streaming
> replication, but not during recovery otherwise
>
> I'm leaning towards 3), it still seems reasonable to not slow down recovery
> when recovering from archive, but the potential for out of disk space
> warrants doing 3.

3) makes sense. But how about 4)?

4) Change it so that checkpoint_segments takes effect in standby mode,
but not during recovery otherwise

This would lessen the time required to restart the standby also in
file-based log shipping case. Of course, there is the tradeoff
between the speed of recovery and the recovery time.

Regards,

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

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