Re: Postgres configuration for 8 CPUs, 6 GB RAM - Mailing list pgsql-performance

Asif:

1. 6GB is pretty small .... once you work through the issues, adding RAM will probably be a good investment, depending on your time-working set curve.

A quick rule of thumb is this:

- if your cache hit ratio is significantly larger than (cache size / db size) then there is locality of reference among queries, and if the hit ratio is less than high 90's percent, then there is a high probablility that adding incremental RAM for caching by the OS and/or PG itself will make things significantly better; this applies to both database-wide averages and individual slow query types.

- Look for long-running queries spilling merges and sorts to disk; if these are a concern then adding RAM and leaving it out of the buffer cache but setting larger work_mem sizes will improve their performance

2. You also need to consider how many queries are running concurrently; limiting the number of concurrent executions to a strict number e.g. by placing the database behind a connection pooler. By avoiding contention for disk head seeking

3. If I/O is a real bottleneck, especially random access, you might consider more drives

4. If the data access is truly all over the place, or you have lots of queries which touch large chucnks of the data, then depending on your budget, a cheap high RAM machine built from a desktop motherboard which will allow you have e.g. 128GB of RAM in low cost modules and thus have the entire DB in RAM is definitely worth considering as a replica server on which to offload some queries. I priced this out at around US$2000 here in America using high quality parts.


These performance tweaks are all of course interrelated ... e.g. if the access patterns are amenable to caching, then adding RAM will reduce I/O load without any further changes, and item 3. may cease to be a problem.

Be careful of the bottleneck issue ... if you're a long way from the performance you need, then fixing one issue will expose another etc. until every part of the system is quick enough to keep up.

Don't forget that your time is worth money too, and throwing more hardware at it is one of many viable strategies.

Cheers
Dave