| From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
|---|---|
| To: | Csaba Nagy <ncslists(at)googlemail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, jd(at)commandprompt(dot)com, Thom Brown <thom(at)linux(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Dave Page <dpage(at)pgadmin(dot)org>, Peter Eisentraut <peter_e(at)gmx(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Configuring synchronous replication |
| Date: | 2010-09-23 13:18:37 |
| Message-ID: | 4C9B53AD.8020002@enterprisedb.com |
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On 23/09/10 15:26, Csaba Nagy wrote:
> Unfortunately it was quite long time ago we last tried, and I don't
> remember exactly what was bottlenecked. Our application is quite
> write-intensive, the ratio of writes to reads which actually reaches the
> disk is about 50-200% (according to the disk stats - yes, sometimes we
> write more to the disk than we read, probably due to the relatively
> large RAM installed). If I remember correctly, the standby was about the
> same regarding IO/CPU power as the master, but it was not able to
> process the WAL files as fast as they were coming in, which excludes at
> least the network as a bottleneck. What I actually suppose happens is
> that the one single process applying the WAL on the slave is not able to
> match the full IO the master is able to do with all it's processors.
There's a program called pg_readahead somewhere on pgfoundry by NTT that
will help if it's the single-threadedness of I/O. Before handing the WAL
file to the server, it scans it through and calls posix_fadvise for all
the blocks that it touches. When the server then replays it, the data
blocks are already being fetched by the OS, using the whole RAID array.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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