> > There is a very good reason: it is the phenomenon of row drift. The
> > master and slave can appear to be in good sync, but often it is not
> > actually the case.
> ... sounds interesting; have you got any document explaining
> this phenomenon? AFAIK, the things that (silently) break
> replication are:
> - non-deterministic functions in statement-based replication
> - hand-made updates on the slave db
> is this enough to justify a *daily* resync?!
I'm definitely no expert on this. All I know is that we used to
frequently experience situations where queries to the slaves would
return different recordsets than the same queries to the masters. Yet by
all other indications the servers were in sync. All the replication
threads were running and the row counts were identical, but the data in
the rows was sometimes different. I asked about this in the list and the
answers I got back were that the phenomenon was called row drift and was
fairly well known and not always easy (or sometimes even possible) to
eliminate because of bad programming practices in some off-the-shelf
applications. At that time, the consensus in the list was that it was
not safe to trust replication slaves for backup purposes. That's when I
came up with the idea of doing an rsync every night, which creates a
slave that is 100% reliable for using as a backup source and also
eliminates problems with row-drift. Since we started using that
technique, we don't get calls from users complaining that their reports
are showing bogus totals and such.
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