Why do I have reading from the swap partition?

Hi Everyone,

I am trying to debug a slowness that is happening on one of my production sites and I would like to ask you for some help.

This is my environment:
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Dedicated server running:
SUSE Linux Enterprise Server 11 (x86_64):
VERSION = 11
PATCHLEVEL = 1

RAM = 16GB

Postgres 9.0.3:

shared_buffers = 4GB
work_mem = 2MB
maintenance_work_mem = 2GB
wal_buffers = 1MB
checkpoint_segments =16
effective_cache_size = 8GB

And this is my scenario:
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I have a table with 16 million records and few indexes for that table.
I also have a query from that table (few filters no joins) that returns 6.000 records. I have the proper indexes and the plan looks good. I don't think the query or the table structure are important that is why I did not post them.

I reboot the server and start postgres:

I run a query first time and it takes ~ 2.5 seconds
I run the same query for the second time and it takes < 1 sec (because it is cached)
All good here.

Now I reboot the server again and start postgres:

I do a select * from a 8 GB table (a different one then the one used in the query). At a point it starts using swap space on disk. Once it starts swapping I still let it run for couple of minutes and the I stop it (CTRL+C).

After that I have 14 GB free memory and in postgres I only have about 30000 buffers used in pg_buffercache, the rest up to 524288 being empty.

If I run my query again then the query takes 60 seconds and I notice reads from the swap partition.

Now my question is why would I have a read from the swap partition when using a table that was not accessed since restart so it is not cached and a have a bunch of free memory and shared buffers?

Could this be a postgres issue?

Thank you in advance,
Ioana