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Re: New replication mode: write

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New replication mode: write
Date: 2012-01-13 12:52:44
Message-ID: CA+U5nMKX=j4CHDC0a=etvEAc=CGp-FcjTyM7ABp_-1m7EM=S1w@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Jan 13, 2012 at 12:27 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:

>> In my patch, the reason I avoided doing WRITE mode (which we had
>> previously referred to as RECV) was that no fsync of the WAL contents
>> takes place. In that case we are applying changes using un-fsynced WAL
>> data and in case of crash this would cause a problem.
>
> My patch has not changed the execution order of WAL flush and replay.
> WAL records are always replayed after they are flushed by walreceiver.
> So, such a problem doesn't happen.

> But which means that transaction might need to wait for WAL flush caused
> by previous transaction even if WRITE mode is chosen. Which limits the
> performance gain by WRITE mode, and should be improved later, I think.

If the WALreceiver still flushes that is OK.

The latency would be smoother and lower if the WALwriter were active.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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