Re: WAL -> Replication

Tom Lane wrote:
> Hannu Krosing <hannu@tm.ee> writes:
> > DB2 can run in two modes 
> > 1) similar to ours, where logs are reused after checkpoints/commits
> > allow it.
> > 2) with log archiving: logs are never reused, but when system determines
> > it no longer needs them, it will hand said log over to archiving process
> > that will archive it (usually do a backup to some other place and then
> > delete it).
> 
> There is in fact the skeleton of support in xlog.c for passing unwanted
> log segments over to an archiver, rather than recycling them.  So far
> no one's done anything with the facility.  I think the main problem is
> the one Bruce cited: because the WAL representation is tied to physical
> tuple locations and so forth, it's only useful to a slave that has an
> *exact* duplicate of the master's entire database cluster.  That's not
> useless, but it's pretty restrictive.
> 
> It could be useful for incremental backup, though I'm not sure how
> efficient it is for the purpose.  WAL logs tend to be pretty voluminous.
> At the very least you'd probably want enough smarts in the archiver to
> strip out the page-image records.

Yes, I think the bottom line is that we would need to add some things to
the WAL file to make archiving the logs work, for either point-in-time
recovery, or replication, both of which we need.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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