John R Pierce wrote:
the commercial cluster software vendors insist on using dedicated connections for the heartbeat messages between the cluster members and insist on having fencing capabilities (for instance, disabling the fiber switch port of the formerly active server and enabling the port for the to-be-activated server). with linux-ha and heartbeat, you're on your own.
This is worth highlighting. As John points out, it's straighforward to build a shared storage implementation using PostgreSQL and either one of the commercial clustering systems or using Linux-HA. And until PostgreSQL gets fully synchronous replication, it's a viable alternate solution for "must not lose a transaction" deployments when the storage used is much more reliable than the nodes.
The hard part of shared storage failover is always solving the "shoot the other node in the head problem", to keep a down node from coming back once it's no longer the active one. In order to do that well, you really need to lock the now unavailable node from accessing the storage at the hardware level--"fencing"--with disabling its storage port being one way to handle that. Figure out how you're going to do that reliably in a way that's integrated into a proper cluster manager, and there's no reason you can't do this with PostgreSQL.
There's a description of the fencing options for Linux-HA at http://www.clusterlabs.org/doc/crm_fencing.html ; the cheap way to solve this problem is to have a UPS that disables the power going to the shot node. Once that's done, you can then safely failover the shared storage to another system. At that point, you can probably even turn back on the power, presuming that the now rebooted system will be able to regain access to the storage during a fresh system start.
-- Greg Smith 2ndQuadrant US Baltimore, MD PostgreSQL Training, Services and Support greg@xxxxxxxxxxxxxxx www.2ndQuadrant.us -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
