Uninitialized pages notices

Our client is running a large-ish database, and has infrequently been
getting the following messages:

Nov  6 04:17:09 dolidb-n1 logger: NOTICE:  Rel pg_type: Uninitialized
page 6 - fixing
Nov  6 04:17:09 dolidb-n1 logger: NOTICE:  Rel pg_type: Uninitialized
page 7 - fixing
Nov  6 04:17:09 dolidb-n1 logger: NOTICE:  Rel pg_type: Uninitialized
page 8 - fixing
Nov  6 04:17:09 dolidb-n1 logger: NOTICE:  Rel pg_type: Uninitialized
page 9 - fixing
Nov  6 04:17:09 dolidb-n1 logger: NOTICE:  Rel pg_type: Uninitialized
page 10 - fixing

This goes on for about 2000 lines.  A google search found Tom Lane
telling someone with this symptom that they might have a serious
hardware problem.  However, _that_ person was getting these messages on
a consistent basis, whereas we haven't seen this message in about 4
months, and then all of a sudden the other night during a nightly vacuum
it popped up.

I'd like to know what this means to our system - what causes an
uninitialized page?  What could cause it to be so infrequent?

Here are our system details:
postgre: 7.2.3 (~4GB in size)
os: Redhat 7.1sbe (Seawolf)
kernel: 2.4.9-12smp
ram: ~1GB ECC

Thanks,
Tim


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