The master currently stores each logfile in a single file, which may have a standard compression applied.
The format is a special case of the netstrings protocol - see http://cr.yp.to/proto/netstrings.txt. The text in each netstring consists of a one-digit channel identifier followed by the data from that channel.
The formatting is implemented in the LogFile class in
buildbot/status/builder.py, and in particular by the merge
method.