Red Hat Directory Server 8.0 Administrator's Guide

4.1. Indexing Performance
While achieving extremely high read performance, in previous versions of Directory Server,
write performance was limited by the number of bytes per second that could be written into the
storage manager's transaction log file. Large log files were generated for each LDAP write
operation; in fact, log file verbosity could easily be 100 times the corresponding number of bytes
changed in the Directory Server. The majority of the contents in the log files are related to index
changes (ID insert and delete operations).
The secondary index structure was separated into two levels in the old design:
• The ID list structures, which were the province of the Directory Server backend and opaque to
the storage manager.
• The storage manager structures (Btrees), which were opaque to the Directory Server
backend code.
Because it had no insight into the internal structure of the ID lists, the storage manager had to
treat ID lists as opaque byte arrays. From the storage manager's perspective, when the content
of an ID list changed, the entire list had changed. For a single ID that was inserted or
deleted from an ID list, the corresponding number of bytes written to the transaction log was the
maximum configured size for that ID list, about 8 kilobytes. Also, every database page on which
the list was stored was marked as dirty, since the entire list had changed.
In the redesigned index, the storage manager has visibility into the fine-grain index structure,
which optimizes transaction logging so that only the number of bytes actually changed need to
be logged for any given index modification. The Berkeley DB provides ID list semantics, which
are implemented by the storage manager. The Berkeley API was enhanced to support the
insertion and deletion of individual IDs stored against a common key, with support for duplicate
keys, and an optimized mechanism for the retrieval of the complete ID list for a given key.
The storage manager has direct knowledge of the application's intent when changes are made
to ID lists, resulting in several improvements to ID list handling:
• For long ID lists, the number of bytes written to the transaction log for any update to the list is
significantly reduced, from the maximum ID list size (8 kilobytes) to twice the size of one ID (4
bytes).
• For short ID lists, storage efficiency, and in most cases performance, is improved because
only the storage manager meditate need to be stored, not the ID list metadata.
• The average number of database pages marked as dirty per ID insert or delete operation is
very small because a large number of duplicate keys will fit into each database page.
4.2. Search Performance
For each entry ID list, there is a size limit that is globally applied to all index keys managed by
Chapter 10. Managing Indexes
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