high (or at least improved) availability and failover

HI,

I've been looking into options for improving the availability of postgres
servers beyond what you get with a monolithic server and RAID. These are, as
far as possible "cheap" options and therefore don't include use of shared
storage and all the good options those systems have.

Here is a very rough summary of my thoughts so far. I havn't actually tried
any of this yet, so if anyone has any thoughts, comments, additions I'd be
glad to hear from you.

regards
iain

Configurations for HA
=====================

Pgpool Only
-----------
Pgpool can detect the failure of a server and failover automatically.

Re-syncing the DBs after a failure will require the databases to be offline
for a file system copy.

Should pgpool run on the db servers or a separate machine(s)?

Performance impact of using the replication facility? Deadlock possibility?

If the machine that pgpool is running on fails, need manual intervention to
start pgpool on another machine.

Can only manage 2 servers at most.

Pgpool and Heartbeat
-----------------
Similar to pgpool only, with the addition that pgpool itself can be failed
over automatically.

Heartbeat and DRDB
---------------
Replication is done at the block level by DRDB.

The database cannot be running on the standby server while it is being
replicated from the master.

Automatic failover is possible as Heartbeat will start the database server
and switch to the standby if the master fails.

A full re-sync of the DBs after a failure doesn't require the master to be
offline.

Failure ofthe standby server typically doesn't require a full re-sync.

Can't do load balancing.

No particular need for pgpool as heartbeat will manage the aliased IP
address

Slony-I only
------------
Relies on triggers in the database, and some administration of the
replication configuration.

Monitors replicated databases and performs automatic failover.

When a failed server is brought back online it will automatically start
catching up with the other servers.

When the master fails, clients can reconnect to the new master without
changing the IP address?

Slony-I and pgpool
---------------
All the features of Slony-I

Some load balancing of SELECT SQL is possible

Need manual intervention if pgpool fails.

Slony-I and pgpool and Heartbeat
----------------------------
All the features of Slony-I with pgpool

Use heartbeat to monitor the machines running pgpool and switch over
automatically if one fails.