TDB: using the jenkins hash for non-persistent tdbs
Stefan (metze) Metzmacher
metze at samba.org
Fri Sep 17 08:25:02 MDT 2010
while doing performance tests with samba/ctdb we found that
locking.tdb processing gets slow for large recursiv directory enumerations.
Even using a very large hash table size doesn't give improvements.
Some hash chains where very long ~300 entries, while most of them
It turned out the default hash algorithm isn't good if the keys
only differ by a few bits.
Rusty will use the jenkins hash
for TDB2 as default, which does better balancing of the keys to the hash
I have a backport of that for usage only for non-persistent tdbs.
The schema will be that we'll be able to detect the usage of the jenkins
on open of an existing tdb. And we'll use it only if a tdb is created
If the jenkins hash is used, we set the rwlocks field in the tdb header
to a magic value.
Current tdb versions expect this to be 0 and fail the open.
That means an current tdbdump won't be able to open such a tdb,
but current Samba with TDB_CLEAR_IF_FIRST, would be able to overwrite
and format it with the current hash function.
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