finding out what's generating WALs

Hi All,

With a 9.4 cluster, what's the best way to find out what's generating 
the most WAL?

I'm looking after a multi-tenant PG 9.4 cluster, and we've started 
getting alerts for the number of WALs on the server.
It'd be great to understand what's generating all that WAL and what's 
likely to be causing any problems.\

More generally, what's number of WALs is "too much"? check_postgres.pl 
when used in nagios format only appears to be able to alert on absolute 
thresholds, does this always make sense? What's a good threshold to 
alert on?

cheers,

Chris