On 10/08/2015 04:47 PM, Michael Paquier wrote:
> On Thu, Oct 8, 2015 at 6:03 PM, Amir Rohan wrote:
>> On 10/08/2015 10:39 AM, Michael Paquier wrote:
>>>> Someone mentioned a daisy chain setup which sounds fun. Anything else in
>>>> particular? Also, it would be nice to have some canned way to measure
>>>> end-to-end replication latency for variable number of nodes.
>>>
>>> Hm. Do you mean comparing the LSN position between two nodes even if
>>> both nodes are not connected to each other? What would you use it for?
>>>
>>
>> In a cascading replication setup, the typical _time_ it takes for a
>> COMMIT on master to reach the slave (assuming constant WAL generation
>> rate) is an important operational metric.
>
> Hm. You mean the exact amount of time it gets to be sure that a given
> WAL position has been flushed on a cascading standby, be it across
> multiple layers. Er, that's a bit tough without patching the backend
> where I guess we would need to keep a track of when a LSN position has
> been flushed. And calls of gettimeofday are expensive, so that does
> not sound like a plausible alternative here to me...
>
Wouldn't this work?
1) start timer
2) Grab pg_stat_replication.sent_location from master
3) pg_switch_xlog() # I /think/ we want this, could be wrong
4) Poll slave's pg_last_xlog_replay_location() until LSN shows up
5) stop timer
Amir
