After snapshotting my master using hot backup to create a workable
slave instance, I created recovery.conf on the slave and tried to get
it to connect to the master and stream WAL.
This led to the message "sorry, too many standbys already", which did
not immediately clue me in as to what I needed to do to fix the
problem. Grepping the source code for the error message revealed that
the problem was that MaxWalSenders was zero. A few seconds further
head-scratching revealed that this was the GUC max_wal_senders, which
I duly increased from 0 to 1, after which it worked.
I think perhaps this error message needs some adjustment. It should
be reasonably possible to guess the name of the GUC that needs
increasing based on the error message, and it currently isn't. Also
I'd vote for making the variable name max_wal_senders rather than
MaxWalSenders, but maybe that's being persnicketty.
...Robert