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Re: Synchronous replication, reading WAL for sending

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Pavan Deolasee <pavan(dot)deolasee(at)enterprisedb(dot)com>
Subject: Re: Synchronous replication, reading WAL for sending
Date: 2008-12-24 10:10:13
Message-ID: 1230113413.4793.1064.camel@ebony.2ndQuadrant (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, 2008-12-24 at 18:31 +0900, Fujii Masao wrote:

> > Frankly, I myself have every doubt
> > that on a relatively high load setup, the standby will not be able
> > keep pace with the primary for two reasons:
> >
> > - Lack of read ahead of data blocks (Suzuki-san's work may help this)
> > - Single threaded recovery
> >
> > But then these are general problems which may impact any log-based replication.
> Right. Completely high load setup is probably impossible. There is
> certainly a price to pay. But, in order to reduce a price as much as
> possible, I think that we should not focus two or more operations
> on single process (walsender) just like single threaded recovery.

I think we may be pleasantly surprised. 

In 8.3 there were two main sources of wait:

* restartpoints
* waiting for archive files

Restartpoints will now be handled by bgwriter, giving probably 20% gain,
plus the WAL data is streamed directly into memory by walreceiver. So I
think the startup process may achieve a better steady state and perform
very quickly.

Suzuki-san's numbers show that full_page_writes = on does not benefit
significantly from having read ahead and we already know that is
effective in reducing the I/O bottleneck during recovery.

If we want to speed up recovery more, I think we'll see the need for an
additional process to do WAL CRC checks. 

 Simon Riggs 
 PostgreSQL Training, Services and Support

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