Re: Having trouble with backups (was: Re: Crash Recovery)

I said:
> Carlos Moreno <moreno@mochima.com> writes:
>> One quick question, Tom, before some general comments and
>> reply to the other messages...  Where would I specify any
>> locks the software wants to do?

> If you are not issuing any explicit "LOCK" SQL commands, then you can
> disregard my theory.

Actually, that's too simple.  Are you creating and dropping tables,
or issuing schema-change commands (such as ADD COLUMN or RENAME)?
All of those things take exclusive locks on the tables they modify.
Ordinary SELECT/INSERT/UPDATE/DELETE operations can run in parallel with
pg_dump, but messing with the database structure is another story.

I guess the real question here is whether your app is actually stopped
dead (as it would be if waiting for a lock), or just slowed to a crawl
(as a performance problem could do).  I cannot tell if your "frozen"
description is hyperbole or literal truth.

One thing that might help diagnose it is to look at the output of ps
auxww (or ps -ef on SysV-ish platforms) to see what all the backends are
currently doing while the problem exists.

            regards, tom lane