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 Debug logs
============

The main debugging tool for Ceph is the dout and derr logging functions.
Collectively, these are referred to as "dout logging."

Dout has several log faculties, which can be set at various log
levels using the configuration management system. So it is possible to enable
debugging just for the messenger, by setting debug_ms to 10, for example.

The dout macro avoids even generating log messages which are not going to be
used, by enclosing them in an "if" statement. What this means is that if you
have the debug level set at 0, and you run this code::

	dout(20) << "myfoo() = " << myfoo() << dendl;


myfoo() will not be called here.

Unfortunately, the performance of debug logging is relatively low. This is
because there is a single, process-wide mutex which every debug output
statement takes, and every debug output statement leads to a write() system
call or a call to syslog(). There is also a computational overhead to using C++
streams to consider. So you will need to be parsimonious in your logging to get
the best performance.

Sometimes, enabling logging can hide race conditions and other bugs by changing
the timing of events. Keep this in mind when debugging.

Performance counters
====================

Ceph daemons use performance counters to track key statistics like number of
inodes pinned. Performance counters are essentially sets of integers and floats
which can be set, incremented, and read using the PerfCounters API.

A PerfCounters object is usually associated with a single subsystem.  It
contains multiple counters. This object is thread-safe because it is protected
by an internal mutex. You can create multiple PerfCounters objects.

Currently, three types of performance counters are supported: u64 counters,
float counters, and long-run floating-point average counters. These are created
by PerfCountersBuilder::add_u64, PerfCountersBuilder::add_fl, and
PerfCountersBuilder::add_fl_avg, respectively. u64 and float counters simply
provide a single value which can be updated, incremented, and read atomically.
floating-pointer average counters provide two values: the current total, and
the number of times the total has been changed. This is intended to provide a
long-run average value.

Performance counter information can be read in JSON format from the
administrative socket (admin_sock). This is implemented as a UNIX domain
socket. The Ceph performance counter plugin for collectd shows an example of how
to access this information. Another example can be found in the unit tests for
the administrative sockets.
