Getting Slow

About six months ago, our normally fast postgres server started
having performance issues. Queries that should have been instant were
taking up to 20 seconds to complete (like selects on the primary key
of a table). Running the same query 4 times in a row would yield
dramatically different results... 1.001 seconds, 5 seconds, 22
seconds, 0.01 seconds, to complete.

At the time we upgraded the hardware and the performance problems
went away. But I did not feel like we had solved the underlying problem.

Now, six months later, the same thing is happening... and I'm kind of
glad because now, I'd like to find out what the real issue is. I'm
just starting to diagnose it so I don't know a lot yet, but what I do
know, I'll share with you here in the hopes of starting off on the
right track.

I've already described the main symptom. Here are some other random
observations:
- The server log shows frequent "archived transaction log file"
entries. Usually once every 10 minutes or so, but sometimes 2 or 3
per minute.
- The server box seems otherwise to be responsive. CPU sits at about
90% idle.
- When queries are especially slow, the server shows a big spike in
read/write activity.
- This morning I did a VACUUM ANALYZE. It seemed to help for 30
minutes or so, but then it was back to being slowish. I'd hate to
schedule these because it feels more like a band-aid. For a long time
we've been doing just fine with autovacuum, so why start scheduling
vacuums now?

Here's info about our configuration. Any advise/pointers would be
much appreciated. Thanks!

Computer: Mac Pro Dual Core Intel
Operating System: Mac OS 10.4.7 Client
Memory: 4GB RAM
Data Drives: 3 drives in a software RAID (internal)
Log/Backup Drive: 1 (the startup disk, internal)

Postgres Version: 8.1.4
Data Size: 5.1 GB
# of Tables: 60
Size of Tables: Most are under 100,000 records. A few are in the
millions. Largest is 7058497.
Average Number of Simultaneous Client Connections: 250

max_connections = 500
shared_buffers = 10000
work_mem = 2048
max_stack_depth = 6000
effective_cache_size = 30000
fsync = on
wal_sync_method = fsync
archive_command = 'cp -i %p /Users/postgres/officelink/wal_archive/%f
</dev/null'
max_fsm_pages = 150000
stats_start_collector = on
stats_row_level = on
log_min_duration_statement = 2000
log_line_prefix = '%t %h '
superuser_reserved_connections = 3
autovacuum = on
autovacuum_naptime = 60
autovacuum_vacuum_threshold = 150
autovacuum_vacuum_scale_factor = 0.00000001
autovacuum_analyze_scale_factor = 0.00000001

sudo pico /etc/rc
sysctl -w kern.sysv.shmmax=4294967296
sysctl -w kern.sysv.shmall=1048576

sudo pico /etc/sysctl.conf
kern.maxproc=2048
kern.maxprocperuid=800
kern.maxfiles=40000
kern.maxfilesperproc=30000

Processes: 470 total, 2 running, 4 stuck, 464 sleeping... 587
threads 13:34:50
Load Avg: 0.45, 0.34, 0.33 CPU usage: 5.1% user, 5.1% sys,
89.7% idle
SharedLibs: num = 157, resident = 26.9M code, 3.29M data, 5.44M
LinkEdit
MemRegions: num = 15307, resident = 555M + 25.5M private, 282M shared
PhysMem: 938M wired, 934M active, 2.13G inactive, 3.96G used,
43.1M free
VM: 116G + 90.1M 1213436(0) pageins, 263418(0) pageouts

PID COMMAND %CPU TIME #TH #PRTS #MREGS RPRVT RSHRD
RSIZE VSIZE
29804 postgres 0.0% 0:03.24 1 9 27 1.27M 245M
175M 276M
29720 postgres 0.0% 0:01.89 1 9 27 1.25M 245M
125M 276M
29714 postgres 0.0% 0:03.70 1 10 27 1.30M 245M
215M 276M
29711 postgres 0.0% 0:01.38 1 10 27 1.21M 245M
107M 276M
29707 postgres 0.0% 0:01.27 1 9 27 1.16M 245M
78.2M 276M
29578 postgres 0.0% 0:01.33 1 9 27 1.16M 245M
67.8M 276M
29556 postgres 0.0% 0:00.39 1 9 27 1.09M 245M
91.8M 276M
29494 postgres 0.0% 0:00.19 1 9 27 1.05M 245M
26.5M 276M
29464 postgres 0.0% 0:01.98 1 9 27 1.16M 245M
88.8M 276M
29425 postgres 0.0% 0:01.61 1 9 27 1.17M 245M
112M 276M
29406 postgres 0.0% 0:01.42 1 9 27 1.15M 245M
118M 276M
29405 postgres 0.0% 0:00.13 1 9 26 924K 245M
17.9M 276M
29401 postgres 0.0% 0:00.98 1 10 27 1.13M 245M
84.4M 276M
29400 postgres 0.0% 0:00.90 1 10 27 1.14M 245M
78.4M 276M
29394 postgres 0.0% 0:01.56 1 10 27 1.17M 245M
111M 276M