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Re: GDQ iimplementation

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: pgsql-cluster-hackers(at)postgresql(dot)org
Subject: Re: GDQ iimplementation
Date: 2010-05-17 21:46:13
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Lists: pgsql-cluster-hackers
Jan, Marko, Simon,

I'm concerned that doing anything about the write overhead issue was 
discarded almost immediately in this discussion.  This is not a trivial 
issue for performance; it means that each row which is being tracked by 
the GDQ needs to be written to disk a minimum of 4 times (once to WAL, 
once to table, once to WAL for queue, once to queue).  That's at least 
one time too many, and effectively doubles the load on the master server.

This is particularly unacceptable overhead for systems where users are 
not that interested in retaining the queue after an unexpected shutdown.

Surely there's some way around this?  Some kind of special 
fsync-on-write table, for example?  The access pattern to a queue is 
quite specialized.

                                   -- Josh Berkus
                                      PostgreSQL Experts Inc.

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