On Fri, Apr 14, 2006 at 08:05:39AM +0200, Terje Elde wrote: > Jim Nasby wrote: > >While working on determining a good stripe size for a database, I > >realized it would be handy to know what the average request size is. > >Getting this info is a simple matter of joining pg_stat_all_tables and > >pg_statio_all_tables and doing some math, but there's one issue I've > >found; it appears that there's no information on how many heap blocks > >were read in by an index scan. Is there any way to get that info? <snip> > Knowing what the average stripe size is can be a good place to start, > but the real question is; which stripe size will allow the majority of > your transactions to be possible to satisfy without having to go to two > spindles? And of course right now there's not a very good way to know that... granted, I can look at the average request size on the machine, but that will include any seqscans that are happening, and for stripe sizing I think it's better to leave that out of the picture unless your workload is heavily based on seqscans. > That said, it's the transactions against disk that typically matter. On > FreeBSD, you can get an impression of this using 'systat -vmstat', and > watch the KB/t column for your drives. On a related note, you know of any way to determine the breakdown between read activity and write activity on FreeBSD? vmstat, systat, iostat all only return aggregate info. :( -- Jim C. Nasby, Sr. Engineering Consultant jnasby@xxxxxxxxxxxxx Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
