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.