Re: High Availability: Hot Standby vs. Warm Standby

Thomas Kellerer wrote:
> The manual lists three possible solutions to HA: shared disk failover,
> file system replication and Warm/Hot Standby. I'm not an admin (nor a
> DBA), so my question might sound a bit stupid: from my point of view
> solutions using shared disk failover of file system replication seem
> to be more reliable in terms of how much data can get lost (and
> possibly the switch over lag)

Yes, but if you try you'll discover that actually getting any shared
disk or file system replication solution setup so that you really do
achieve less failover loss than the file shipping approach will be
expensive, complicated, fragile in its own way, and just generally a
pain to pull off. The fundamental problem with shared storage for
example is how to keep a note that's failed from try to reassume being
the master when it comes back. Doing that well requires hardware
support aimed at that specific use case.

Meanwhile, file shipping for Warm Standby use requires nothing special
at all except some modest software setup. It's comparatively simple to
setup, validate, and keep going on any hardware capable of running the
database. This is why shared storage and the like isn't the only
obvious solution even though it's technically capable of losing less
transactions; you'll discover that keeping from losing that last little
bit of data when there's a crash turns out to be quite expensive.
Whether it worth it or not depends on the value of your data and whether
it can be retained at some higher level when this happens instead.

--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us