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pg_dump lock timeout

From: daveg <daveg(at)sonic(dot)net>
To: pgsql-patches(at)postgresql(dot)org
Cc: hari(at)efrontier(dot)com
Subject: pg_dump lock timeout
Date: 2008-05-11 11:30:47
Message-ID: 20080511113047.GV5673@sonic.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Attached is a patch to add a commandline option to pg_dump to limit how long
pg_dump will wait for locks during startup.

The intent of this patch is to allow pg_dump to fail if a table lock cannot
be taken in a reasonable time. This allows the caller of pg_dump to retry or
otherwise correct the situation, without having locks held for long periods,
and without pg_dump having a long window during which catalog changes can
occur.

It works by setting statement_timeout to the user specified delay during
the startup phase where it is taking access share locks on all the tables.
Once all the locks are taken, it sets statement_timeout back to the default.
If a lock table statement times out, the dump fails with the statement timed
out error.

The orginal motivation was a client who runs heavy batch workloads and uses
truncate table and other DML in long transactions. This has created some
unhappy interaction scenarios with pg_dump:

  - pg_dump ends up waiting hours on a DML table lock that is part of a long
    transaction. Once the lock is released, pg_dump runs only to find
    some table later in the list has been dropped. So pg_dump fails.

  - pg_dump waits on a lock while holding access share locks on most of the
    tables. Other processes that want to do DML wait on pg_dump. After a
    while, large parts of the application are blocked while pg_dump waits
    on locks. Eventually the operations staff notice that pg_dump is
    blocking production and kill the dump.

Please have a look and consider it for merging.

Thanks

-dg

-- 
David Gould
If simplicity worked, the world would be overrun with insects.

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