HP-UX Directory Server 8.1 administrator guide

Because maintaining large ID lists in memory can affect performance, the All IDs Threshold set
a limit on how large a single entry ID list could get. When a list reaches a certain predetermined
size, the search would treat it as if the index contained the entire directory.
The difficulty in setting the All IDs Threshold hurt performance. If the threshold was too low,
too many searches examined every entry in the directory. If it was too high, too many large ID
lists had to be maintained in memory.
The problems addressed by the All IDs Threshold are no longer present because of the efficiency
of entry insertion, modification, and deletion in the Berkeley DB design. The All IDs Threshold
is removed for database write operations, and every ID list is now maintained accurately.
Because loading a long ID list from the database can significantly reduce search performance,
the configuration parameter, nsslapd-idlistscanlimit, sets a limit on the number of IDs
that are read before a key is considered to match the entire primary index.
nsslapd-idlistscanlimit is analogous to the All IDs Threshold, but it only applies to the
behavior of the server's search code, not the content of the database.
When the server uses indexes in the processing of a search operation, it is possible that one index
key matches a large number of entries. For example, consider a search for
objectclass=inetorgperson in a directory that contained one million inetorgperson
entries. When the server reads the inetorgperson index key in the objectclass index, it
finds one million matching entries. In cases like this, it is more efficient simply to conclude in
the index lookup phase of the search operation processing that all the entries in the database
match the query. This causes subsequent search processing to scan the entire database content,
checking each entry as to whether it matches the search filter. The time required to fetch the
index keys is not worthwhile; the search operation is likely to be processed more efficiently by
omitting the index lookup.
In Directory Server, when examining an index, if more than a certain number of entries are found,
the server stops reading the index and marks the search as unindexed for that particular index.
The threshold number of entries is called the idlistscanlimit and is configured with the
nsslapd-idlistscanlimit configuration attribute. The default value is 4000, which is
designed to give good performance for a common range of database sizes and access patterns.
Typically, it is not necessary to change this value. However, in rare circumstances it may be
possible to improve search performance with a different value. For example, lowering the value
will improve performance for searches that will otherwise eventually reach the default limit of
4000. This might reduce performance for other searches that benefit from indexing. Conversely,
increasing the limit could improve performance for searches that were previously hitting the
limit. With a higher limit, these searches could benefit from indexing where previously they did
not.
For more information on search limits for the server, see “Overview of the searching algorithm”.
11.5.3 Backwards compatibility and migration
While current versions of Directory Server can support the old database design, only the new
design is supported for this and later releases of Directory Server.
Upon startup, the server will read the database version from the DBVERSION file, which contains
the text Netscape-ldbm/6.2 (old database version), Netscape-ldbm/7.1 (new database
format), or bdb/4.2/libback-ldbm (new database format). If the file indicates that the old
format is used, then the old code is selected for the database. Because the DBVERSION file stores
everything per back-end, it is possible to have different database formats for different individual
back-ends, but the old database format is not recommended.
All databases must be migrated to Directory Server 8.0 when the system is upgraded. Migration
is supported for Directory Server 6.x versions, and for releases earlier than version 6.x, dump
the databases be dumped, and install Directory Server fresh. Migrating databases is covered in
the HP-UX Directory Server installation guide.
11.5 Managing indexes 469