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Re: WAL logs multiplexing?

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Dmitry Panov <dmitry(at)tsu(dot)tula(dot)ru>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-general(at)postgresql(dot)org,pgsql-hackers(at)postgresql(dot)org
Subject: Re: WAL logs multiplexing?
Date: 2005-12-29 11:58:06
Message-ID: 1135857486.2964.745.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackers
On Thu, 2005-12-29 at 10:47 +0300, Dmitry Panov wrote:
> On Wed, 2005-12-28 at 11:05 -0500, Tom Lane wrote:
> > Dmitry Panov <dmitry(at)tsu(dot)tula(dot)ru> writes:
> > > Yes, but if the server has crashed earlier the script won't be called
> > > and if the filesystem can't be recovered the changes will be lost. My
> > > point is the server should write into both (or more) files at the same
> > > time.
> > 
> > As for that, I agree with the other person: a RAID array does that just
> > fine, and with much higher performance than we could muster.
> > 
> 
> BTW, I found something related in the TODO:
> http://momjian.postgresql.org/cgi-bin/pgtodo?pitr
> 
> I think both approaches have the right to exist, but I prefer my because
> it looks more straightforward, it insures up-to-date recovery (no
> delays) and it reduces the traffic (as the partial logs have to be
> transferred in full by the proposed "archive_current_wal_command"). The
> only drawback is performance.

Simply replicating pg_xlog might be worthwhile for the truly paranoid,
since it does help in the situation that you lose the RAID unit with
your pg_xlog on it. But this facility is already available via hardware
replication facilities, so I see no reason to build it into the DBMS.

Replicating pg_xlog to NFS would not work very well performance wise and
has some major undefined behaviour in most failure modes, so I would
never do that.

However, there is a case to be made for "continuous xlog record
archival" which could get closer to 0% data loss in the event of
failure, though with higher performance hit than current PITR. I'll look
into that some more - but no promises.

Best Regards, Simon Riggs



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