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How And Why To Use TDB Tracing
==============================
You can trace all TDB operations, using TDB_TRACE. It is not complete
(error conditions which expect to the logged will not always be traced
correctly, so you should set up a logging function too), but is designed
to collect benchmark-style traces to allow us to optimize TDB.
Note: tracing is not efficient, and the trace files are huge: a
traverse of the database is particularly large! But they compress very
well with rzip (http://rzip.samba.org)
How to gather trace files:
--------------------------
1) Uncomment /* #define TDB_TRACE 1 */ in tdb_private.h.
2) Rebuild TDB, and everything that uses it.
3) Run something.
Your trace files will be called <tdbname>.trace.<pid>. These files
will not be overwritten: if the same process reopens the same TDB, an
error will be logged and tracing will be disabled.
How to replay trace files:
--------------------------
1) For benchmarking, remember to rebuild tdb with #define TDB_TRACE commented
out again!
2) Grab the latest "replace_trace.c" from CCAN's tdb module (tools/ dir):
http://ccan.ozlabs.org/tarballs/tdb.tar.bz2
3) Compile up replay_trace, munging as necessary.
4) Run replay_trace <scratch-tdb-name> <tracefiles>...
If given more than one trace file (presumably from the same tdb)
replay_trace will try to figure out the dependencies between the operations
and fire off a child to run each trace. Occasionally it gets stuck, in
which case it will add another dependency and retry. Eventually it will
give a speed value.
replay_trace can intuit the existence of previous data in the tdb (ie.
activity prior to the trace(s) supplied) and will prepopulate as
necessary.
You can run --quiet for straight benchmark results, and -n to run multiple
times (this saves time, since it need only calculate dependencies once).
Good luck!
Rusty Russell <rusty@rustcorp.com.au>
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