serious performance problem

Hi everybody,

I'm having a serious performance problem. We're using PostgreSQL
(7.1.3) for data collection (appr. 3 000 000 new records per day).
Every night all records older than 3 days are deleted, after that a
vacuumdb is done. The table has about 7-8 M records and is about 2,5GB
size on the file-system. I noticed that the time consumed by vaccum
increases from about 5 mins to 11 mins now within the last 5 weeks
which I thought this happens because of increased tables over the last
weeks on the system.

The whole system worked well for several weeks now. Starting this
night everything slowed down. Simple SQL commands (Select
date(dt),sum(bytes) from table where source between 'xxx.xxx.xxx.xxx'
and 'xxx.xxx.xxx.xxx' and dt >= '10-30-2001' and dt < '10-31-2001
00:00:00' group by date(dt)) which are done within a few seconds
before take minutes now to compute... There were no changes on the
system the last times. The file-system has no errors.

PostgreSQL is installed on a RedHat 7.1 with Kernel 2.4.2-2. The
hardware is a PIII-500 with 256MB RAM and 10GB HD (IDE).

Does anyone has a idea what the reason could be and how it could be
fixed?

Thanks in advance!

Martin