While only running pg_dump, postgresql performs writes inside base/ directory. How can we stop this?

We are dumping a 4TB postgres database using pg_dump. The source is on a
copy-on-write snapshot filesystem, and the target is an external drive.

While the pg_dump is running we get about 4-5MB/sec writes on the
filesystem where postgresql data is (iostat), and specifically inside the
base/ directory.

Since the only query running on this DB is the COPY initiated from pg_dump
which only reads data, who is writing data to postgres datafiles?

We need to find a way to make postgres stop writing because the source
copy-on-write filesystem gets filled up before the dump is finished.

Postgresql version is 8.4, vacuum is not running (autovacuum = off), OS is
debian 6.