HP OpenVMS Systems Documentation
Volume Shadowing for OpenVMS
|Example 3-1 Nodes Not Registered to Use Volume Shadowing|
%LICENSE-E-NOAUTH, DEC VOLSHAD use is not authorized on this node -LICENSE-F-NOLICENSE, no license is active for this software product -LICENSE-I-SYSMGR, please see your system manager
For more information about the License Management Facility, refer to the OpenVMS Operating System Software Product Description (SPD 25.01.xx).
You can also consult the OpenVMS License Management Utility Manual.
After you register the volume shadowing PAK, you must set the shadowing
parameters on each node where you want to enable shadowing.
3.2 Volume Shadowing Parameters
Table 3-1 lists the system parameters that are required to specify the use of Volume Shadowing for OpenVMS and the system parameters you can use to tailor the shadowing software on your system. These parameters were introduced with OpenVMS Version 7.1, except for ALLOCLASS, which was introduced earlier, and SHADOW_MAX_UNIT, which was introduced in OpenVMS Version 7.3. The term dynamic in Table 3-1 means that the active value can be changed on a running system. For more information about setting system parameters, see the OpenVMS System Manager's Manual.
OpenVMS Version 7.3 introduced four write bitmap system parameters (described in Table 3-3) and the volume shadowing system parameter SHADOW_MAX_UNIT. These system parameters support the shadowing minicopy operation, which is described in Chapter 7.
|ALLOCLASS||Specifies the device allocation class for the system. When using Volume Shadowing for OpenVMS, a nonzero value is required.||0--255||0||No|
|SHADOWING||A value of 2 enables volume shadowing. See Table 3-2 for a description of parameter values.||0, 2 1||0||No|
|SHADOW_MAX_COPY||Limits the number of concurrent merge or copy operations on a given node.||0--200||4||Yes|
|SHADOW_MBR_TMO||Controls the amount of time the system tries to fail over physical members of a shadow set.||
|SHADOW_MAX_UNIT||Specifies the maximum number of shadow sets that can exist on a node. Dismounted shadow sets, unused shadow sets, and shadow sets with no write bitmaps allocated to them are included in this total.||10--10,000||100 on VAX; 500 on Alpha||No|
|SHADOW_SYS_DISK||Allows system disk to be a shadow set and, optionally, enables a minimerge to occur. If a minimerge is enabled, the system must also be configured for writing to a nonshadowed, nonsystem disk of your choice.||0, 1, 4097 1||0||Yes|
|SHADOW_SYS_TMO||Controls the amount of time members of a system disk shadow set have to return to the set.||
|SHADOW_SYS_UNIT||Contains the virtual unit number of the system disk.||0--9999||0||No|
|SHADOW_SYS_WAIT||This parameter applies only to shadow sets that are currently mounted in the cluster. Controls the amount of time a booting system will wait for all members of a mounted system disk shadow set to become available.||
This section provides guidelines for using volume shadowing parameters.
The ALLOCLASS parameter is used to specify an allocation class that forms part of a device name. The purpose of allocation classes is to provide unique and unchanging device names. When using Volume Shadowing for OpenVMS on a single system or on an OpenVMS Cluster system, a nonzero allocation class value is required for each physical device in the shadow set. For more information about using allocation classes, see the OpenVMS Cluster Systems manual.
Table 3-2 describes these settings in detail.
Shadowing is not enabled.
This is the default value.
Enables host-based shadowing.
This setting provides shadowing of all disks that are located on a standalone system or on an OpenVMS Cluster system. Set SHADOWING to 2 on every node that will mount a shadow set, including satellite nodes.
The SHADOW_MAX_COPY parameter controls how many parallel copy and merge operations are allowed on a given node. (Copy and merge operations are described in Chapter 6.) This parameter provides a way to limit the number of copy and merge operations in progress at any one time.
The value of SHADOW_MAX_COPY can range from 0 to 200. The default value is specific to the OpenVMS version. You can determine the default value by looking at the parameter setting. When the value of the SHADOW_MAX_COPY parameter is 4, and you mount five multivolume shadow sets that all need a copy operation, only four copy operations can proceed. The fifth copy operation must wait until one of the first four copies completes.
Consider the following when choosing a value for the SHADOW_MAX_COPY parameter:
For example, the default value of 4 may be too high for a small node. (In particular, satellite nodes should have SHADOW_MAX_COPY set to a value of 0.) Too low a value for SHADOW_MAX_COPY unnecessarily restricts the number of operations your system can effectively handle and extends the amount of time it takes to merge all of the shadow sets.
SHADOW_MAX_COPY is a dynamic parameter. Changes to the parameter affect only future copy and merge operations; current operations (pending or already in progress) are not affected.
The SHADOW_MAX_UNIT specifies the maximum number of shadow sets that can exist on a node. The important thing to note about this value is that any shadow set that has been created, regardless of whether it is in use, is included in this total. Because this is not a dynamic system parameter, you should be very careful when determining the value to use. If you need to change this parameter, you must reboot the system. Furthermore, any attempt to mount more shadow sets than the value specified will fail.
Note that this parameter does not affect the naming of shadow sets. For example, with the default value of 100, a device name such as DSA999 is still valid.
The SHADOW_MBR_TMO parameter controls the amount of time the system tries to fail over physical members of a shadow set before removing them from the set. SHADOW_MBR_TMO is a dynamic parameter that you can change on a running system.
With the SHADOW_MBR_TMO parameter, you specify the number of seconds, from 1 to 65,535, during which recovery of a shadow set member is attempted.
The value of SHADOW_MBR_TMO should not exceed the value of the parameter MVTIMEOUT.
If you specify zero, a default delay is used. The default delay is specific to the version of OpenVMS running on your system. For shadow sets in an OpenVMS Cluster configuration, the value of SHADOW_MBR_TMO should be set to the same value on each node.
Determining the correct value for SHADOW_MBR_TMO is a trade-off between rapid recovery and high availability. If rapid recovery is required, set SHADOW_MBR_TMO to a low value. This ensures that failing shadow set members are removed from the shadow set quickly and that user access to the shadow set continues. However, removal of shadow set members reduces data availability and, after the failed member is repaired, a full copy operation is required when it is mounted back into the shadow set.
If high availability is paramount, set SHADOW_MBR_TMO to a high value. This allows the shadowing software additional time to regain access to failed members. However, user access to the shadow set is stalled during the recovery process. If recovery is successful, access to the shadow set continues without the need for a full copy operation, and data availability is not degraded. Setting SHADOW_MBR_TMO to a high value may be appropriate when shadow set members are configured across LANs that require lengthy bridge recovery time.
Shadowing uses a timer to adhere to the number of seconds specified by the SHADOW_MBR_TMO parameter. For directly connected SCSI devices that have been powered down or do not answer to polling, the elapsed time before a device is removed from a shadow set can take several minutes.
If you enable a minimerge of the system disk, you must also configure your system to write a dump to a nonshadowed, nonsystem disk of your choice. This is known as dump off system disk (DOSD). For more information on DOSD, see the OpenVMS System Manager's Manual: Tuning, Monitoring, and Complex Systems manual.
During the booting process, you can use this parameter on the first node in the cluster to boot and to create a specific shadow set. If the proposed shadow set is not currently mounted in the cluster, use this parameter to extend the time a booting system will wait for all former members of the system disk shadow set to become available.
The second use of this parameter comes into effect once the system successfully mounts the shadow set and begins normal operations. Just as the SHADOW_MBR_TMO parameter controls the time the operating system waits for failing members of an application disk shadow set to rejoin the shadow set, the SHADOW_SYS_TMO parameter controls how long the operating system will wait for failing members of a system disk shadow set. All nodes using a particular system disk shadow set should have their SHADOW_SYS_TMO parameter equal to the same value, after normal operations begin. Therefore, after booting, this parameter applies only to members of the system disk shadow set.
The default value is OpenVMS version specific. You can set a range of up to 65,535 seconds if you want the system to wait longer than the default for all members to join the shadow set.
The SHADOW_SYS_UNIT parameter is an integer value that contains the virtual unit number of the system disk. The default value is 0. The maximum value allowed is 9999. This parameter is effective only when the SHADOW_SYS_DISK parameter has a value of 1. This parameter must be set to the same value on all nodes that boot off a particular system disk shadow set. SHADOW_SYS_UNIT is not a dynamic parameter.
Use the SHADOW_SYS_WAIT parameter to extend the time a booting system
will wait for all current members of a mounted system disk shadow set
to become available to this node. SHADOW_SYS_WAIT is a dynamic
parameter that you can change on a running system (for debugging
purposes only). The shadow set must already be mounted by at least one
other cluster node for this parameter to take effect. The default value
is 256 seconds. Change this parameter to a higher value if you want the
system to wait more than the 256-second default for all members to join
the shadow set. This parameter has a range of 1 through 65,535 seconds.
3.3 Write Bitmap System Parameters
Starting with OpenVMS Version 7.3, system parameters are available for managing update traffic between a master write bitmap and its corresponding local write bitmaps in an OpenVMS Cluster system. Another 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 Table 3-3 and described in detail in Section 7.9. These system parameters support the minicopy operation (see Chapter 7).
|WBM_MSG_INT||In single-message mode, the time interval between assessment of the most suitable write bitmap message mode. In buffered-message mode, the maximum time a message waits before it is sent.||msec||10||-1||10|
|WBM_MSG_UPPER||The upper threshold for the number of messages sent during the test interval that will initiate buffered-message mode.||msgs/interval||0||-1||100|
|WBM_MSG_LOWER||The lower threshold for the number of messages sent during the test interval that will initiate single-message mode.||msgs/interval||0||-1||10|
|WBM_OPCOM_LVL||Controls whether write bitmap messages are provided to the operator console: 0 means messages are turned off; 1 means messages are provided when write 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|