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56 lines
2.5 KiB
ReStructuredText
56 lines
2.5 KiB
ReStructuredText
============
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Debug logs
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============
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The main debugging tool for Ceph is the dout and derr logging functions.
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Collectively, these are referred to as "dout logging."
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Dout has several log faculties, which can be set at various log
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levels using the configuration management system. So it is possible to enable
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debugging just for the messenger, by setting debug_ms to 10, for example.
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The dout macro avoids even generating log messages which are not going to be
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used, by enclosing them in an "if" statement. What this means is that if you
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have the debug level set at 0, and you run this code::
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dout(20) << "myfoo() = " << myfoo() << dendl;
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myfoo() will not be called here.
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Unfortunately, the performance of debug logging is relatively low. This is
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because there is a single, process-wide mutex which every debug output
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statement takes, and every debug output statement leads to a write() system
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call or a call to syslog(). There is also a computational overhead to using C++
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streams to consider. So you will need to be parsimonious in your logging to get
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the best performance.
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Sometimes, enabling logging can hide race conditions and other bugs by changing
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the timing of events. Keep this in mind when debugging.
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Performance counters
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====================
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Ceph daemons use performance counters to track key statistics like number of
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inodes pinned. Performance counters are essentially sets of integers and floats
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which can be set, incremented, and read using the PerfCounters API.
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A PerfCounters object is usually associated with a single subsystem. It
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contains multiple counters. This object is thread-safe because it is protected
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by an internal mutex. You can create multiple PerfCounters objects.
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Currently, three types of performance counters are supported: u64 counters,
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float counters, and long-run floating-point average counters. These are created
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by PerfCountersBuilder::add_u64, PerfCountersBuilder::add_fl, and
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PerfCountersBuilder::add_fl_avg, respectively. u64 and float counters simply
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provide a single value which can be updated, incremented, and read atomically.
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floating-pointer average counters provide two values: the current total, and
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the number of times the total has been changed. This is intended to provide a
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long-run average value.
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Performance counter information can be read in JSON format from the
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administrative socket (admin_sock). This is implemented as a UNIX domain
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socket. The Ceph performance counter plugin for collectd shows an example of how
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to access this information. Another example can be found in the unit tests for
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the administrative sockets.
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