Volume Shadowing Parameters
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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 in OpenVMS Version 7.1, except for:
ALLOCLASS, introduced prior to OpenVMS Version 7.1
SHADOW_MAX_UNIT, introduced in OpenVMS Version 7.3
SHADOW_HBMM_RTC, SHADOW_PSM_DLY, and SHADOW_REC_DLY, introduced in OpenVMS Version 8.2
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 HP OpenVMS System Manager's Manual.
OpenVMS Version 7.3 also introduced four bitmap system parameters, which are described in Table 3-4. These system parameters support the host-based minicopy operation, described in Chapter 7, and the host-based minimerge (HBMM) operation, described in Chapter 8.
Table 3-1 Volume Shadowing Parameters
Parameter | Function | Range | Default | Dynamic |
|---|---|---|---|---|
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. | 0, 2[1] | 0 | No |
SHADOW_ENABLE | Special parameter reserved for HP use. | - | - | - |
SHADOW_HBMM_RTC | Specifies, in seconds, how frequently each shadow set has its modified block count compared with its reset threshold. If the modified block count exceeds the reset threshold, the bitmap for that shadow set is zeroed. | 60–65535 | 150 | Yes |
SHADOW_MAX_COPY | Limits the number of concurrent merge or copy operations on a given node. | 0–200 | 4 | Yes |
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 bitmaps allocated to them are included in this total. | 10–10,000 | 500 on Alpha | No |
| SHADOW_MBR_TMO | Controls the amount of time the system tries to fail over physical members of a shadow set. | 1–65,535 seconds | 120 | Yes |
SHADOW_PSM_DLY | Allows the system manager to adjust the delay that Shadowing adds automatically when a copy or merge operation is needed on a shadow set that is mounted on many systems. The Shadowing facility attempts to perform the operation on a system that has a local connection to all the shadow set members. Shadowing implements the copy or merge operation by adding a time delay based on the number of shadow set members that are MSCP-served to the system. No delay is added for local members. Therefore, a system with all locally accessible shadow set members usually performs the copy or merge before a system on which one or more members is served and is therefore delayed. | 0–65535 seconds | 30 seconds for each MSCP -served shadow set member | No |
SHADOW_REC_DLY | Governs the system behavior after a system failure or after a shadow set is aborted. The value of this parameter is added to the value of the RECNXINTERVAL parameter to determine how long a system waits before it attempts to manage a merge or copy operation on any shadow sets that it has mounted. | 0–65535 seconds | 20 | Yes |
SHADOW_SITE_ID | On an Alpha system, allows a system manager to define a site value, which volume shadowing uses to determine the best device to perform reads, thereby improving performance. | 1–255 | No | Yes |
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 non-shadowed, non-system disk of your choice. | 0, 1, 4097[1] | 0 | Yes |
SHADOW_SYS_TMO | Applies to members of a shadowed system disk only. Has two distinct uses: At system boot time when this is the first node in the cluster to boot and to create this specific shadow set. Use this parameter to extend the time a booting system waits for all former members of the shadowed system disk to join the shadow set. After the system successfully mounts the shadowed system disk and begins normal operations. Then, this parameter controls the time the operating system waits for other members of the shadowed system disk to join the shadow set. Use the SHADOW_MBR_TMO parameter to control the time the operating system waits for members of an application disk. | 1–65,535 seconds | 120 | Yes |
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 waits for all members of a mounted system disk shadow set to become available. | 1–65,535 seconds | 480 | Yes |
[1] All other values are reserved for use by HP. | ||||
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 HP OpenVMS Cluster Systems manual.
The SHADOWING parameter enables or disables volume shadowing on your system, as shown in Table 3-2.
Table 3-2 SHADOWING Parameter Settings
| Setting | Effect |
|---|---|
0 | Shadowing is not enabled. This is the default value. |
2 | 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 mounts a shadow set, including satellite nodes. |
SHADOW_HBMM_RTC (Integrity servers and Alpha)
SHADOW_HBMM_RTC is used to specify, in seconds, how frequently each shadow set on this system has its modified block count compared with its reset threshold. If the modified block count exceeds the reset threshold, the bitmap for that shadow set is zeroed. This comparison is performed for all shadow sets mounted on the system that have HBMM bitmaps. The reset threshold is specified by the RESET_THRESHOLD keyword in the /POLICY qualifier of the SET SHADOW command. When the comparison is made, the modified block count might exceed the reset threshold by a small increment or by a much larger amount. The difference depends on the write activity to the volume and the setting of this parameter.
The default setting of SHADOW_HBMM_RTC is 150 seconds.
You can view the reset threshold setting and the modified block count, since the last reset, in the SHOW SHADOWcommand display. For guidelines on setting the reset threshold values and a sample SHOW SHADOW display, see “Considerations for Setting a Bitmap RESET_THRESHOLD Value”. For a SHOW SHADOW display that includes a modified block count greater than the reset threshold value, see the Example 9 for SHOW SHADOW in the HP OpenVMS DCL Dictionary.
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 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 must 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 number of shadow sets that can exist on a node and determines the memory reserved for the bitmap for each shadow set. (See “Memory Requirements”.) 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 must be very careful when determining the value to use. If you have to change this parameter, you must reboot the system.
The default value for OpenVMS Alpha systems is 500.
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 | CAUTION: Any MOUNT command that attempts to create more shadow sets than the maximum specified for the node fails. |
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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.
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 | NOTE: The value of SHADOW_MBR_TMO must not exceed the value of the parameter MVTIMEOUT. |
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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 must 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.
The use of default settings for certain system parameters may lead to the occasional removal of shadow set members (systems that are using Volume Shadowing for OpenVMS) that are configured for multi-path support. Therefore, when configuring multi-path shadow sets using Volume Shadowing for OpenVMS, follow the recommendations shown in Table 3-3.
Table 3-3 System Parameter Settings for Multipath Shadow Sets
System Parameter | Recommended Setting |
|---|---|
MSCP_CMD_TMO | 60 as a minimum. The value of 60 is appropriate for most configurations. Some configurations may require a higher setting. |
SHADOW_MBR_TMO | At least 3 x MSCP_CMD_TMO |
SHADOW_SYS_TMO | At least 3 x MSCP_CMD_TMO |
MVTIMEOUT | At least 4 x SHADOW_MBR_TMO |
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| NOTE: The recommended setting for MVTIMEOUT, as shown in Table 3-3, represents a doubling of an earlier recommendation published for OpenVMS Alpha Version 7.3. |
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To modify SHADOW_MBR_TMO for an existing shadow set member, see the SET SHADOW/RECOVERY_OPTIONS=DELAY_PER_SERVED_MEMBER=n command, which is described in “Managing Copy and Merge Operations (Integrity servers and Alpha)”.
SHADOW_PSM_DLY allows the system manager to adjust the delay that Shadowing adds automatically when a copy or merge operation is needed on a shadow set that is mounted on many systems.
The Shadowing facility attempts to perform the operation on a system that has a local connection to all the shadow set members. Shadowing implements the copy or merge operation by adding a time delay based on the number of shadow set members that are MSCP-served to the system. No delay is added for local members. Therefore, a system with all locally accessible shadow set members usually performs the copy or merge before a system on which one or more members is served and is therefore delayed.
When a shadow set is mounted on a system, the value of SHADOW_PSM_DLY is used as the default shadow set member recovery delay for that shadow set. To modify SHADOW_PSM_DLY for an existing shadow set, see the SET SHADOW/RECOVERY_OPTIONS=
DELAY_PER_SERVED_MEMBER=n command, which is described in “Managing Copy and Merge Operations (Integrity servers and Alpha)”.
SHADOW_PSM_DLY is a static parameter; its range is 0 to 65535 seconds. The default value is 30 seconds for each MSCP served shadow set member.
SHADOW_REC_DLY governs the system behavior after a system failure or after a shadow set is aborted. The value of the SHADOW_REC_DLY parameter is added to the value of the RECNXINTERVAL parameter to determine how long a system waits before it attempts to manage a merge or copy operation on any shadow sets that it has mounted.
SHADOW_REC_DLY can be used to predict the systems that can perform recovery operations in an OpenVMS Cluster. This is done by setting lower values of SHADOW_REC_DLY on systems that are preferred to handle recovery operations and higher values of SHADOW_REC_DLY on the remaining systems.
SHADOW_REC_DLY is a dynamic parameter; its range is 0 to 65535 seconds. The default value is 20 seconds.
For more information about controlling which systems perform the merge or copy operations, see “Controlling Which Systems Manage Merge and Copy Operations”.
A SHADOW_SYS_DISK parameter value of 1 enables shadowing of the system disk. A value of 0 disables shadowing of the system disk. A value of 4097 enables a minimerge. The default value is 0.
If you enable a minimerge of the system disk, you must also configure your system to write a dump to a non-shadowed, non-system disk of your choice. This is known as dump off system disk (DOSD). For more information on DOSD, see the HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems.
In addition, you must specify a system-disk, shadow-set virtual unit number with the SHADOW_SYS_UNIT system parameter, unless the desired system disk virtual unit number is DSA0.
You can use the SHADOW_SYS_TMO parameter in two ways: during the booting process and during normal operations. SHADOW_SYS_TMO is a dynamic parameter that you can change on a running system.
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 waits 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 waits for failing members of a system disk shadow set. All nodes using a particular system disk shadow set must 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, which must be used when the SHADOW_SYS_DISK parameter is set to 1, contains the virtual unit number of the system disk.
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 waits 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.