pg_dump and write locks

We have an issue with running pg_dump while a database is under
reasonably heavy update load. This is 7.4.5 on Solaris 9/intel.

The observed behavior was that a pg_dump running with nothing else going
on takes a couple of minutes, but when we are running some system tests
that do heavy updates to a selection of application tables, it appears
that pg_dump blocks until the update run is done. This didn't make sense
because everything I can find about pg_dump indicates that it should
only be taking read locks, so I don't see why it should be blocked.

We looked at pg_locks, and saw that the pg_dump process was acquiring
locks like:

14764 | ExclusiveLock | 124576072 | COPY public.stats (id,
description, lastsavedate, lastsaveuser) TO stdout;

(we are using pg_dump with -Fc) If COPY is taking a ROW EXCLUSIVE lock,
then that would explain why we are seeing contention, but I don't
understand why COPY is locking at that level. Is there a better way to
run pg_dump against a database with a lot of update activity?

Thanks.

- DAP
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David Parker Tazz Networks