Re: Sleepycat and hash functions

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

At 11:56 AM 12/14/2002, Hallvard B Furuseth wrote:
>http://www.openldap.org/faq/index.cgi?file=756 says one can use BDB's
>db_dump utility for on-line backups while slapd is running.
>However, db_dump's manpage says:
>
>  Dumping and reloading Hash databases that use user-defined hash
>  functions will result in new databases that use the default hash
>  function.  Although using the default hash function may not be optimal
>  for the new database, it will continue to work correctly.
>
>Have anyone noticed whether or not such backed-up databases are slower
>than "original" databases?  Since slapd precalculates hashes and uses a
>no-op hash function in back-bdb/dbcache.c, I suppose that means all data
>are hashed twice - once by slapd and once by Sleepycat.
>
>The manpage continues:
>
>  Dumping and reloading Btree databases that use user-defined prefix or
>  comparison functions will result in new databases that use the default
>  prefix and comparison functions.  In this case, it is quite likely
>  that the database will be damaged beyond repair permitting neither
>  record storage or retrieval.
>
>Since db_dump is recommended, I take it back-bdb does not use Btree
>databases?

It does... but not with a user-defined comparison function.
User-defined comparison function is only defined for index
databases (which use DB_HASH).  So, if they are slower, one
can always recreate them using slapindex(8).

Kurt