On Thu, Jun 24, 2010 at 02:43:33PM +0200, Janning wrote: > Hi, > > at the moment we encounter some performance problems with our database server. > > We have a 12 GB RAM machine with intel i7-975 and using > 3 disks "Seagate Barracuda 7200.11, ST31500341AS (1.5 GB)" > One disk for the system and WAL etc. and one SW RAID-0 with two disks for > postgresql data. Our database is about 24GB. > > Our munin graph reports at 9:00 a clock writes of 3000 blocks per second and > reads of about 1000 blocks per second on our disk which holds the data > directories of postgresql (WAL are on a different disk) > > 3000 blocks ~ about 3 MB/s write > 1000 blocks ~ about 1 MB/s read > > At the same time we have nearly 50% CPU I/O wait and only 12% user CPU load > (so 4 of 8 cpu cores are in use for io wait) > > We know, its a poor man disk setup (but we can not find a hoster with rather > advanced disk configuration at an affordable price). Anyway, we ran some tests > on it: > > > # time sh -c "dd if=/dev/zero of=bigfile bs=8k count=3000000 && sync" > 3000000+0 records in > 3000000+0 records out > 24576000000 bytes (25 GB) copied, 276.03 s, 89.0 MB/s > > real 4m48.658s > user 0m0.580s > sys 0m51.579s > > # time dd if=bigfile of=/dev/null bs=8k > 3000000+0 records in > 3000000+0 records out > 24576000000 bytes (25 GB) copied, 222.841 s, 110 MB/s > > real 3m42.879s > user 0m0.468s > sys 0m18.721s > > > > Of course, writing large chunks is quite a different usage pattern. But I am > wondering that writing 3MB/s and reading 1 MB/s seams to be a limit if i can > run a test with 89 MB/s writing and 110MB/s reading. > > Can you give some hints, if this numbers seems to be reasonable? > > kind regards > Janning > Yes, these are typical random I/O versus sequential I/O rates for hard drives. Your I/O is extremely under-powered relative to your CPU/memory. For DB servers, many times you need much more I/O instead. Cheers, Ken -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
