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Interpreting pg_stat_replication values

From: Alexander Fortin <alexander(dot)fortin(at)gmail(dot)com>
To: pgsql-admin(at)postgresql(dot)org
Subject: Interpreting pg_stat_replication values
Date: 2012-01-16 09:25:37
Message-ID: 4F13ED11.6080001@gmail.com (view raw or flat)
Thread:
Lists: pgsql-admin
Hi folks,

We've got to monitor replication status (and lag) of a few 9.1.2 
replicas and I'm trying to use values in master's pg_stat_replication to 
do so.

The documentation [1] on postgresql.org doesn't explains exactly the 
internals, especially what the *_location values means. What I'm doing 
right now is to convert replay_location from 64 bit hex to integer and 
trace the growing trend, and looking at that I think it's quite 
impossible to be a WAL filename because the growing rate is way too fast 
(average 100k per second, with spikes of 8G per second).

Anyway, the whole idea is to check "distance" between the actual value 
for the master (still have to figure out where is that) and the replicas 
replay_location, and alert if that value is too high compared to our 
average trends. If you've got better ideas to spare, don't hesitate ;)

Thanks for your time


[1] 
http://www.postgresql.org/docs/9.1/static/monitoring-stats.html#MONITORING-STATS-VIEWS

-- 
Alexander Fortin
http://about.me/alexanderfortin/

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