MarkLogic 9 Product Documentation

Database Merge Configuration Help

A Database is a collection of Forests.

A Forest consists of Stands.

Each Stand is a set of XML fragments, and is implemented by a set of compressed binary files contained within a sub-directory of the forest directory.

The following are the configuration options:

• merge priority specifies the CPU scheduler priority at which merges should run. The settings are:
  • normal specifies the same CPU scheduler priority as for requests.
  • lower specifies a lower CPU scheduler priority than for requests.
  Merges always run with normal priority on forests with more than 16 stands.
• merge max size specifies the maximum size, in megabytes, of a stand that will result from a merge. If a stand grows beyond the specified size, it will not be merged. If two stands would be larger than the specified size if merged, they will not be merged together. If you set this to smaller sizes, large merges (which may require more disk and CPU resources) will be prevented. Set this to 0 to allow any sized stand to merge. The default is 32768 (32G), which provides a good balance between keeping the number of stands low and preventing merges from needing large amounts of free disk space. Use care when setting this to a non-zero value lower than the default value, however, as this can prevent merges which are ultimately required for the system to maintain performance levels and to allow optimized updates to the system.
• merge min size specifies the minimum number of fragments that stands can contain. Two or more Stands with fewer than this number of fragments are automatically merged.
• merge min ratio specifies the minimum ratio between the number of stand fragments. Stands with a fragment count below this ratio relative to all smaller stands are automatically merged with the smaller stands. Specify a positive integer for the merge min ratio.
• merge timestamp specifies the timestamp stored on merged stands. This is used for point-in-time queries, and determines when space occupied by deleted fragments and old versions of fragments may be reclaimed by the database. If a fragment is deleted or updated at a time after the merge timestamp, then the old version of the fragment is retained for use in point-in-time queries. Set this to 0 (the default) to let the system reclaim the maximum amount of disk space during merge activities. A setting of 0 will remove all deleted and updated fragments when a merge occurs. Set this to 1 before loading or updating any content to create a complete archive of the changes to the database over time. Set this to the current timestamp (by clicking the current timestamp button) to preserve all versions of content from this point on. Set this to a negative number to specify a window of timestamp values, relative to the last merge, at ten million ticks per second. The timestamp is a number maintained by MarkLogic Server that increments every time a change occurs in any of the databases in a system (including configuration changes from any host in a cluster). To set to the current timestamp, click the current timestamp button; the timestamp is displayed in in red until you press OK to activate the timestamp for future merges. For details on point-in-time queries, see the Application Developer's Guide.

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• Merge blackout periods allow you to specify times when merges are disabled. To specify a merge blackout period, click the Create tab and specify when you want the blackout to occur. You can make it a recurring blackout period, or specify a one-time blackout period. Use caution when setting large blackout periods when there are significant updates occurring on the system; merges are a normal part of the self-tuning mechanism of the database, and disabling them completely or for long periods of time can cause performance degradation.

• Use the Create tab to create a new merge blackout period. You can create recurring periods or a one-time blackout period.
• Use the Configure tab to configure merge parameters for this database.