Netscape NETSCAPE DIRECTORY SERVER 6.02, Руководство администратора

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Managing Indexes
Chapter 10 Managing Indexes 367
Managing Indexes
Each index that the directory uses is composed of a table of index keys and
matching entry ID lists. This entry ID list is used by the directory to build a list of
candidate entries that may match a client application’s search request (see “About
Indexes,” on page 341 for details).
For each entry ID list there is a size limit as specified in the
nsslapd-allidsthreshold
attribute. This size limit is globally applied to all index
keys managed by the server and is logically called All IDs Threshold. When the
size of an individual ID list reaches this limit, the server replaces that entry ID list
with an All IDs token.
The All IDs token causes the server to assume that all directory entries match the
index key. In effect, the All IDs token causes the server to behave as if no index was
available for that type of search. The directory assumes that some other aspect of
the search request will allow the server to narrow its candidate list before
processing the request.
The following sections examine the benefits and drawbacks of the All IDs
mechanism. They also give advice for the tuning of the All IDs Threshold.
Benefits of the All IDs Mechanism
The All IDs mechanism is an important mechanism for improving search
performance in those cases where the search results would be most or all directory
entries (for example, searches such as
cn=*
). By assuming that all entry IDs are
returned Directory Server:
• Does not have to maintain infinitely increasing entry ID lists, thus minimizing
your Directory Server’s disk space usage
• Does not have to load unnecessarily large entry ID lists into memory in
response to search requests that result in all directory entries anyway, thus
increasing search performance by reducing large disk reads
• Does not require large amounts of RAM to hold in memory unnecessarily large
entry ID lists
Drawbacks of the All IDs Mechanism
Performance problems can occur if the All IDs threshold is set either too low (this is
the most common problem) or too high for your directory’s size.