HP UX Red Hat Direry Server Software manual Indexing Performance, Search Performance

Chapter 10. Managing Indexes

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

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