[Pacemaker] PGSQL resource promotion issue

[Takatoshi MATSUO]

Hi Steven

Sorry for late reply

2013/3/29 Steven Bambling <smbambling at arin.net>:
> Taskatoshi/Rainer thanks so much for the quick responses and clarification.
>
> In response to the rep_mode being set to sync.
>
> If the master is running the monitor check as low as every 1s, then its updating the nodes with the "new" master preference in the event that the current synchronous replica couldn't be reached and the postgres service then selected the next node in the synchronous_standby_names list to perform they synchronous transaction with.
>
> If you are doing multiple transactions a second then doesn't it become possible for the postgres service to switch it synchronous replication replica ( from node2 to node3 for instance ) and potentially fail ( though I think the risk seems small ) before the monitor function is invoke to update the master preference?
>
> In this case you've committed a transaction(s) and reported it back to your application that it was successful, but when the new master is promoted it doesn't have the committed transactions because it is located on the other replica  ( and the failed master ).  Basically you've lost these transactions even though they were reported successful.

Yes !
I didn't consider this situation.

>
> The only way I can see getting around this would be to compare the current xlog locations on each of the remaining replicas, the promoting the one that meets your business needs.
>         1. If you need to have greater data consistency.
>                 - promote the node that has the furtherest log location even IF they haven't been replayed and there is some "recovery" period.
>
>         2. If you need to have greater "up time"
>                 - promote the node that has the furtherest log location, taking into account the replay lag
>                         - promote the node that has the furthest head or near furthest ahead log location and the LESS replay lag.

How do slaves get "up time" ?
I think slaves can't know the replay lag.

> Does this even seem possible with a resource agent or is my thinking totally off?

Method 1 and 2 may cause data loss.
If you can accept data loss, you use "rep_mode=async".
It's about the same as method 1.


How about refraining from switching synchronous replication replica to avoid
data loss to set one node into "synchronous_standby_names" ?


Thanks,
Takatoshi MATSUO