From: Janning <ml@planwerk6.de>
To: pgsql-performance@postgresql.org
Subject: Write performance
Date: Thu, 24 Jun 2010 14:43:33 +0200
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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