Write Image Journaling
InterSystems IRIS safeguards database updates by using a two-phase technique called write image journaling. In this process, the Write daemon creates the write image journal file (WIJ) during InterSystems IRIS startup. Database updates are initially written from memory to this transitional journal, IRIS.WIJ, and then to the database. If the system crashes during the second phase, the updates can be reapplied upon recovery.
The following topics are covered in greater detail:
Two-Phase Write Protocol
InterSystems IRIS maintains application data in databases whose structure enables fast and efficient searches and updates. Generally, when an application updates data, InterSystems IRIS must modify multiple blocks in the database structure to reflect the change. InterSystems IRIS uses a two-phase write protocol to protect the integrity of the database in the event of a crash that might otherwise lead to a subset of those block writes getting lost. This protocol operates as follows:
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In the first phase, InterSystems IRIS writes the updated blocks to the WIJ. Once all of the updates are durably written, InterSystems IRIS sets a flag in the WIJ to indicate that there are blocks to restore. Then the second phase begins.
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In the second phase, InterSystems IRIS writes the same set of blocks recorded in the WIJ to the database on disk. When these blocks are durably written, InterSystems IRIS clears the flag to indicate that there are no blocks to restore from the WIJ.
When InterSystems IRIS starts, it automatically checks the WIJ and runs a recovery procedure if it detects that an abnormal shutdown occurred. When the procedure completes successfully, the internal integrity of the database is restored. InterSystems IRIS also runs WIJ recovery following a shutdown as a safety precaution to ensure that database can be safely backed up.
Write Image Journal (WIJ) Settings
By default, the WIJ file is named IRIS.WIJ and resides in the system manager directory, install-dir/mgr, where install-dir is the installation directory. You may specify a different location for this file, as well as a target file size, from the Journal Settings page of the Management Portal. To do so:
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Navigate to the Journal Settings page of the Management Portal (System Administration > Configuration > System Configuration > Journal Settings).
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Enter the new location of the WIJ in the Write image journal directory box and click Save. The name must identify an existing directory on the system and may be up to 63 characters long. If you edit this setting for a clustered instance, restart InterSystems IRIS to apply the change; no restart is necessary for a standalone instance.
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Enter the target size for the WIJ at the Target size for the wij (MB) (0=not set) prompt. The default of zero allows the WIJ to grow as needed but does not reserve space for this; entering a non-zero value reserves the specified space on the storage device.
For information about the two settings described, which are included in the instance’s iris.cpf file, see targwijsz and wijdir in the [config] section of the Configuration Parameter File Reference.
