On 06.10.2010 11:49, Fujii Masao wrote:
On Wed, Oct 6, 2010 at 5:17 PM, Heikki Linnakangas
<heikki.linnakan...@enterprisedb.com> wrote:
Sure, but it's not the synchronous aspect that increases availability. It's
the replication aspect, and we already have that. Making the replication
synchronous allows zero data loss in case the master suddenly dies, but it
comes at the cost of availability.
Yep. But I mean that the synchronous aspect is helpful to increase the
availability of the system which requires no data loss. In asynchronous
replication, when the master goes down, we have to salvage the missing
WAL for the standby from the failed master to avoid data loss. This would
take very long and decrease the availability of the system which doesn't
accept any data loss. Since the synchronous doesn't require such a salvage,
it can increase the availability of such a system.
In general, salvaging the WAL that was not sent to the standby yet is
outright impossible. You can't achieve zero data loss with asynchronous
replication at all.
If we want only no data loss, we have only to implement the wait-forever
option. But if we make consideration for the above-mentioned availability,
the return-immediately option also would be required.
In some (many, I think) cases, I think that we need to consider availability
and no data loss together, and consider the balance of them.
If you need both, you need three servers as Simon pointed out earlier.
There is no way around that.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
