JONESD at er6s1.eng.ohio-state.edu
Fri Oct 31 20:09:29 GMT 2003
The zip file http://www.er6.eng.ohio-state.edu/~jonesd/samba/vms_tdb2.zip
contains a replacement for vms_tdb.c/.h. The single source file vms_tdb.c has
been split into 2 files (vms_tdb2.c and vms_tdb_isam.c) and tdb.h reorganizes
the TDB_CONTEXT structure.
Major changes for the rewrite of VMS TDB file:
- The TDB_CONTEXT structure was changed to include a function dispatch table
for and isolate the VMS-specific structures. One set of functions, defined
in vms_tdb2.c, handles 'internal' memory-resident tdbs and another set,
defined in vms_tdb_isam.c handles file-based persistent tdbs.
- The TDB file now stores format version information in the key name
field of the primary key's key descriptor. The original format, indicated
by a blank on missing description, will be known as version 0 and is
still supported by this module. The new format will known as version 1
and the description has the form "TDBHASH.1/n" where 'n' in the number of
bits in the hash mask. The RMS characteristics have not changed between
the 2 formats.
- Version 1 uses a different hash function which includes limiting the
result value to 'n' bits (default 9). When using this few bits, the
gdbm hash used in version 0 does not produce a good distribution for the
keys used in unexpected.tdb.
- If tdb_open() must create a new file, the target directory is searched for
a vms_tdb_default.conf configuration file. If found, this text file is
scanned to determine the number of bits to use in the hash mask. Each
line of the file is either a comment or a line consisting of the filename
followed by a space and an integer representing the number of bits.
- Version 1 deletes entries by truncating the record to 4 bytes (the primary
key) rather than performing an RMS delete. A subsequent insert of a record
with the same key will reuse this deleted entry. This change avoids the
problem of the tdb file filling up it's data buckets with RFA's of deleted
records. Keeping the deleted records may add some overhead to tdb_traverse
operations, but this overhead is limited by the relatively small range of
- The RAB now specifies manual unlocking for the read records rather than
automatic locking. In general, the lookup sub-function accumulates the
locks on all records it reads with that same hash key and then releases
them together as a group. This helps avoid some race conditions when
adding a new record.
The new tdb functions seem to pass the tdbtorture test OK.
David L. Jones | Phone: (614) 292-6929
Ohio State University | Internet:
140 W. 19th St. Rm. 231a | jonesd at er6s1.eng.ohio-state.edu
Columbus, OH 43210 | vman+ at osu.edu
Disclaimer: I'm looking for marbles all day long.
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