limited disk space

We need to run a server (7.4.5, Solaris 9/Intel) in an environment with
a defined limit on disk size. We know basically the set of data we will
be working with, and can size the disk space accordingly, but there will
be a fair amount of update churn in the data.

We are running autovacuum, but we still seem to be running out of disk
space in our long running tests. The testers claim that the disk usage
is not going down with a VACUUM FULL, but I have not verified that
independently.

Given that our "real" dataset is fairly fixed, and the growth in the
database size is due to updates, I'm wondering if there is a way that I
can allocate enough disk space at the outset to allow the database to
have a large enough "working set" of free pages so that once it reaches
a certain threshold it doesn't have to grow the database files anymore.

I'm also wondering how WAL settings may affect the disk usage. It's not
an option to place the logs on a separate device in this case, so I
imagine I want to limit the size there, too.

Is anybody running postgres in a similar constrained environment, or are
there any general tips on controlling disk usage that somebody could
point me to?

Thanks.

- DAP
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David Parker Tazz Networks (401) 709-5130