Bitmap System Parameters

OpenVMS Version 7.3 introduced four system parameters for managing minicopy bitmap messages. The four parameters apply equally to managing HBMM bitmap messages. Three parameters are used to manage update traffic between a master bitmap and its corresponding local bitmaps in an OpenVMS Cluster system. The fourth parameter controls whether bitmap system messages are sent to the operator console and, if they are to be sent, the volume of messages. System parameters are dynamic; they can be changed on a running system. Table 3-4 lists the bitmap system parameters.

The bitmap system parameters check if the messages are buffered and then packaged in a single System Communications Services (SCS) message to update the master bitmap or whether each message is sent immediately. The system parameters are used to set the upper and lower thresholds 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 at a time in individual SCS messages to the node with the master bitmap. This is known as single-message mode.

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

Table 3-4 Bitmap System Parameters

Parameter	Meaning	Unit	Min	Max^[1]	Default
WBM_MSG_INT	In single-message mode, the time interval between assessment of the most suitable bitmap message mode. In buffered-message mode, the maximum time (in milliseconds) that a message waits before it is sent.	msec	10^[2] 1^[3]	-1^[2] 100^[3]	10^[2] 7^[3]
WBM_MSG_UPPER	The upper threshold for the number of messages sent during the test interval (calculated in 100 millisecond windows) that initiates buffered-message mode.	msgs/interval	0	-1	80
WBM_MSG_LOWER	The lower threshold for the number of messages sent during the test interval (calculated in 100 millisecond windows) that initiates single-message mode.	msgs/interval	0	-1	20
WBM_ OPCOM_ LVL	Controls whether bitmap messages are provided to the operator console: 0 means messages are turned off; 1 means messages are provided when bitmaps are started, deleted, and renamed, and when the SCS message mode (buffered or single) changes; 2 means that all messages for a setting of 1 are provided along with detailed messages for debugging purposes.	n/a	0	2	1
^[1]The maximum value of -1 corresponds to the maximum positive value that can be represented by a longword. ^[2]For OpenVMS Version 8.3 and earlier. ^[3]For OpenVMS Version 8.4.

Setting System Parameters

To set or modify volume shadowing parameters, edit the [SYSn.SYSEXE]MODPARAMS.DAT file or the appropriate AUTOGEN include file. After editing the file, execute SYS$UPDATE:AUTOGEN as described in the HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems. If you have an OpenVMS Cluster system, ensure that the system parameters are updated on each node. Example 3-2 illustrates a MODPARAMS.DAT file that includes assignment statements to set shadowing parameters.

Example 3-2 MODPARAMS.DAT File

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! Volume Shadowing Parameters:
SHADOWING=2             ! Enables phase II shadowing

SHADOW_SYS_DISK=1       ! Enables system disk shadowing

SHADOW_SYS_UNIT=7       ! Specifies 7 as the virtual unit number 
                          of the system disk

SHADOW_MAX_COPY=4       ! Specifies that 4 parallel copies can occur at one time

SHADOW_MBR_TMO=120      ! Allows 120 seconds for physical members to fail over
                        ! before removal from the shadow set

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See the HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems for complete information about invoking AUTOGEN and specifying the appropriate command qualifiers to perform the desired AUTOGEN operations.

Displaying System Parameters

It is sometimes useful to use the SYSGEN command SHOW to display the values of system parameters. You do not need special privileges to invoke the SYSGEN utility. You can use either a qualifier or the name of a system parameter with the SHOW command, or you can use the SHOW/ALL command to display information about all system parameters. (Enter HELP SHOW at the SYSGEN> prompt for more information about the SHOW command.) The following example illustrates how you can check the current default, minimum, and maximum values for the SHADOWING parameter.

$ MCR SYSGEN
SYSGEN>  SHOW SHADOWING
Parameter Name            Current    Default     Min.       Max.   Unit  Dynamic
--------------            -------    -------   -------    -------  ----  -------
SHADOWING                       2          0         0          2 Coded-valu
SYSGEN>