Simon,
On 09/24/2010 12:11 AM, Simon Riggs wrote:
> As I keep pointing out, waiting for an acknowledgement from something
> that isn't there might just take a while. The only guarantee that
> provides is that you will wait a long time. Is my data more safe? No.
By now I agree that waiting for disconnected standbies is useless in
master-slave replication. However, it makes me wonder where you draw the
line between just temporarily unresponsive and disconnected.
> To get zero data loss *and* continuous availability, you need two
> standbys offering sync rep and reply-to-first behaviour. You don't need
> standby registration to achieve that.
Well, if your master reaches the false conclusion that both standbies
are disconnected and happily continues without their ACKs (and the idiot
admin being happy about having boosted database performance with
whatever measure he recently took) you certainly don't have no zero data
loss guarantee anymore.
So for one, this needs a big fat warning that gets slapped on the
admin's forehead in case of a disconnect.
And second, the timeout for considering a standby to be disconnected
should rather be large enough to not get false negatives. IIUC the
master still waits for an ACK during that timeout.
An infinite timeout doesn't have either of these issues, because there's
no such distinction between temporarily unresponsive and disconnected.
Regards
Markus Wanner