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.