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System Management Features

4.12 New System Parameters

By default VCC_READAHEAD is 1, which means that the Extended File Cache

can use read-ahead caching. The Extended File Cache detects when a ﬁle is being

read sequentially in equal-sized I/Os, and fetches data ahead of the current read,

so that the next read instruction can be satisﬁed from cache.

To stop the Extended File Cache from using read-ahead caching, set

VCC_READAHEAD to 0.

Changing the value of VCC_READAHEAD affects volumes currently mounted on

the local node, as well as volumes mounted in the future.

Readahead I/Os are totally asynchronous from user I/Os and only take place if

sufﬁcient system resources are available.

VCC_READAHEAD is a DYNAMIC parameter.

4.12.12 WBM_MSG_INT

WBM_MSG_INT is one of three system parameters that are available for

managing the update trafﬁc between a master write bitmap and its corresponding

local write bitmaps in an OpenVMS Cluster system. (Write bitmaps are used

by the volume shadowing software for minicopy operations.) The others are

WBM_MSG_UPPER and WBM_MSG_LOWER. These parameters set the interval

at which the frequency of sending messages is tested and also set an upper and

lower threshold that determine whether the messages are grouped into one SCS

message or are sent one by one.

In single-message mode, WBM_MSG_INT is the time interval in milliseconds

between assessments of the most suitable write bitmap message mode. In single-

message mode, 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. If

the writes sent by a remote node reach an upper threshold of messages during a

speciﬁed interval, single-message mode switches to buffered-message mode.

In buffered-message mode, WBM_MSG_INT is the maximum time a message

waits before it is sent. In buffered-message mode, the messages 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.

The minimum value of WBM_MSG_INT is 10 milliseconds. The maximum value

is -1, which corresponds to the maximum positive value that a longword can

represent. The default is 10 milliseconds.

WBM_MSG_INT is a DYNAMIC parameter.

4.12.13 WBM_MSG_LOWER

WBM_MSG_LOWER is one of three system parameters that are available for