Ceph

3775 introduced the observation that logs can fill very quickly and bury a small root disk.
Our documentation could perhaps

1) not recommend such high levels as it does in the example, and
2) make it some clearer just how fast logs can fill up, even without any I/O load

For instance, the example given:

[global]
        debug ms = 1

[mon]
        debug mon = 20
        debug paxos = 20
        debug auth = 20

[osd]
        debug osd = 20
        debug filestore = 20
        debug journal = 20
        debug monc = 20

[mds]
        debug mds = 20
        debug mds balancer = 20
        debug mds log = 20
        debug mds migrator = 20

mon = 20 will generate a pile of monitor messages by itself; this combination was said
to generate 1-2GB of log per hour,which is excessive. Recommend:

1) showing as an example the settings that we typically recommend for a first "what the hell is going on" type escalation past default levels
2) recommending watching log directories carefully after changing log levels to get an idea of just how much data is coming how fast
3) maybe some notes about the fact that the log level is shown on each log line, and sometimes the subsystem, so you can perhaps tune down just below messages that seem unuseful