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Media recovery

 

Media recovery re-creates objects from information recorded in a linear log. For example, if an object file is inadvertently deleted, or becomes unusable for some other reason, media recovery can re-create it. The information in the log required for media recovery of an object is called a media image. Media images can be recorded manually, using the rcdmqimg command, or automatically in some circumstances.

A media image is a sequence of log records containing an image of an object from which the object itself can be re-created.

The first log record required to re-create an object is known as its media recovery record; it is the start of the latest media image for the object. The media recovery record of each object is one of the pieces of information recorded during a checkpoint.

When an object is re-created from its media image, it is also necessary to replay any log records describing updates performed on the object since the last image was taken.

Consider, for example, a local queue that has an image of the queue object taken before a persistent message is put onto the queue. In order to re-create the latest image of the object, it is necessary to replay the log entries recording the putting of the message to the queue, as well as replaying the image itself.

When an object is created, the log records written contain enough information to completely re-create the object. These records make up the object’s first media image. Subsequently, at each shutdown, the queue manager records media images automatically as follows:

Media images can also be recorded manually using the rcdmqimg command, described in rcdmqimg (record media image). This command writes a media image of the WebSphere MQ object. Once this has been done, only the logs that hold the media image, and all the logs created after this time, are needed to re-create damaged objects. The benefit of doing this depends on such factors as the amount of free storage available, and the speed at which log files are created.

 

Parent topic:

Recovering damaged objects


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