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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.
+
+Dout is implemented mainly in common/DoutStreambuf.cc
+
+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.