replay_location meaning in pg_stat_replication

Hi all,

I'm just wondering what the numbers really mean. The output is "2FA/B3AFB890", for example. Do these numbers represent something meaningful I can use? Bytes, offsets within files, what? How do you monitor the difference between write_location and replay_location? It seems obvious the values are hex. Anybody has code they want to share?

Section 25.2.5.2 in the 9.1 docs states:

You can retrieve a list of WAL sender processes via the pg_stat_replication (http://www.postgresql.org/docs/9.1/static/monitoring-stats.html#MONITORING-STATS-VIEWS-TABLE) view. Large differences between pg_current_xlog_location and sent_location field might indicate that the master server is under heavy load, while differences between sent_location and pg_last_xlog_receive_location on the standby might indicate network delay, or that the standby is under heavy load.

What is a "large difference" in this context?

Thanks!
François