Re: Configuring synchronous replication

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