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System Parameters for Managing Write Bitmap Messages and Shadow Set Limit  



System parameters are available for managing the update traffic between a master write bitmap and its corresponding local write bitmaps in an OpenVMS Cluster system. Another new system parameter controls whether write bitmap system messages are sent to the operator console and if they are to be sent, the volume of messages. These system parameters are dynamic, that is, they can be changed on a running system. They are shown in Write Bitmap System Parameters.

In addition, a new volume shadowing system parameter, SHADOW_MAX_UNIT, is provided for specifying the maximum number of shadow sets that can exist on a node. This parameter is described in Volume Shadowing Parameters.

The system parameters for managing write bitmap message traffic control whether the messages are buffered and then packaged in a single SCS message to update the master write bitmap or whether each one is sent immediately. The system parameters are used to set the upper and lower threshholds of message traffic and a time interval during which the traffic is measured.

The writes issued by each remote node are, by default, sent one by one in individual SCS messages to the node with the master write bitmap. This is known as single-message mode.

If the writes sent by a remote node reach an upper threshhold of messages during a specified interval, single-message mode switches to buffered-message mode. In buffered-message mode, the messages (up to nine) are collected for a specified interval and then sent in one SCS message. During periods of increased message traffic, grouping multiple messages to send in one SCS message to the master write bitmap is generally more efficient than sending each message separately.


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