Interpreting pg_stat_replication values

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/