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HP OpenVMS Version 8.2 New Features and Documentation Overview

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6.4.2 Merge Resulting from Mount Verification Timeout

A shadow set that enters mount verification and either times out or aborts mount verification will enter a merge state if the following conditions are true:

  • There are outstanding write I/O requests in the shadow driver's internal queues on the system or systems on which it has timed out.
  • The shadow set is mounted on other systems in the cluster.

The system on which the mount verification timed out (or aborted mount verification) notifies the other systems on which the shadow set is mounted that a merge operation is needed, and then it will disable the shadow set. (It does not dismount it.)

For example, if a shadow set is mounted on eight systems and mount verification times out on two of them, those two systems check their internal queues for write I/O. If any write I/O is found, the shadow set will need to be merged.

6.4.3 Merge Resulting from Use of SET SHADOW/DEMAND_MERGE

The SET SHADOW/DEMAND_MERGE command initiates a merge of a specified shadow set or of all shadow sets. This qualifier is useful if the shadow set was created with the INITIALIZE/SHADOW command without the use of the /ERASE qualifier.

The SET SHADOW command was introduced in OpenVMS Alpha Version 7.3-2. For more information about using SET SHADOW/DEMAND_MERGE, refer to HP OpenVMS DCL Dictionary and to the HP Volume Shadowing for OpenVMS manual.

6.4.4 Comparison of Merge and Minimerge Operations

In a full merge operation, the members of a shadow set are compared with each other to ensure that they contain the same data. This is done by performing a block-by-block comparison of the entire volume. This can be a very lengthy procedure.

A minimerge operation can be significantly faster. By using information about write operations that were logged in volatile controller storage or in a write bitmap on an OpenVMS system, volume shadowing merges only those areas of the shadow set where write activity occurred. This avoids the need for the entire volume scan that is required by full merge operations, thus reducing consumption of system I/O resources.

Prior to the introduction of HBMM, minimerge was controller-based and available only on the HSJ, HSC, and HSD controllers.

6.5 Overview of HBMM

HBMM depends on bitmaps and policies to provide the information required for minimerge operations. Depending on your computing environment, one HBMM policy, a DEFAULT policy that you specify, might be sufficient.

Before you can use HBMM for recovery of a shadow set, the following conditions must be true:

  • An HBMM policy exists.
  • An HBMM policy is associated with a shadow set.
  • The shadow set is mounted on one or more systems that are specified in the HBMM policy.

When a policy is associated with a shadow set and the shadow set is mounted on several systems, bitmaps specific to that shadow set are created.

The systems selected from the master list, as specified in the HBMM policy definition, can perform a minimerge operation because they possess the master bitmaps. All other systems on which the shadow set is mounted possess a local bitmap for each master bitmap.

6.5.1 Bitmaps: Master and Local

For a given bitmap, there is exactly one master version on some system in the cluster and a local version on every other system that has the associated shadow set mounted. A minimerge operation can occur only on a system with a master bitmap. A shadow set can have up to six HBMM master bitmaps. Multiple master bitmaps for the same shadow set are equivalent but they do have different bitmap IDs.

The following example shows two master bitmaps for DSA12, one on RAIN and one on SNOW, each with a unique bitmap ID:


Device         BitMap    Size     Percent      Type of   Master  Active
Name            ID     (Bytes)   Populated     Bitmap     Node
DSA12:        00020007     8364        0%     Minimerge  RAIN      Yes
              00010008     8364        0%     Minimerge  SNOW     Yes

If only one master bitmap exists for the shadow set, and the system with the master bitmap fails or is shut down, the bitmap is gone; that is, the remaining local versions are automatically deleted. Local bitmaps cannot be used for recovery.

If multiple master bitmaps were created for the shadow set and at least one remains, that master bitmap can be used for recovery. HP recommends the use of multiple master bitmaps, especially for multiple-site cluster systems. Multiple master bitmaps increase the likelihood of an HBMM operation rather than a full merge in the event of a system failure.

Bitmaps require additional memory. The calculation is based on the shadow set volume size. For every gigabyte of storage of a shadow set mounted on a system, 2 KB of bitmap memory is required on that system for each bitmap. For example, a shadow set with a volume size of 200 GB of storage and 2 bitmaps uses 800 KB of memory on every system on which it is mounted.

6.5.2 HBMM Policies

A policy specifies the following attributes for one or more shadow sets:

  • Names of systems that are eligible to host a master bitmap.
  • Number of systems that will host a master bitmap (not to exceed six). If this number is omitted, the first available six systems of the systems you specified are selected.
  • Threshold (in 512-byte blocks) at which the bitmaps are reset. If omitted, the threshold defaults to 50,000 blocks.

You can assign almost any name to a policy. However, the reserved names DEFAULT and NODEFAULT have specific properties that are described in Section 6.7. You can also create a policy without a name and assign it to a specific shadow set. An advantage of a named policy is that it can be reused by specifying only its name.

Multiple policies can be created to customize the minimerge operations in a cluster.

You use the SET SHADOW/POLICY command with HBMM specific qualifiers to define, assign, deassign, and delete policies and to enable and disable HBMM on a shadow set. SET SHADOW/POLICY is the only user interface for specifying HBMM policies. You cannot use the MOUNT command to define a policy. You can define a policy before the shadow set is mounted. (Policies can be associated with shadow sets in other ways as well, as described in Section 6.7.)

6.6 HBMM Policy Specification Syntax

An HBMM policy specification consists of a list of HBMM policy keywords enclosed with parentheses. The HBMM policy keywords are MASTER_LIST, COUNT, and RESET_THRESHOLD. Of the three keywords, only MASTER_LIST must be specified. If COUNT and RESET_THRESHOLD are omitted, default values are supplied. (For examples of policy specifications, see Section 6.9.1 and HP OpenVMS DCL Dictionary.)

The use of these keywords and the rules for specifying them are described in this section.


The MASTER_LIST keyword is used to identify a set of systems as candidates for a master bitmap. The system-list value can be a single system name; a parenthesized, comma-separated list of system names; or the asterisk (*) wildcard character. For example:


When the system list consists of a single system name or the wildcard character, parentheses are optional.

An HBMM policy must include at least one MASTER_LIST. Multiple master lists are optional. If a policy has multiple master lists, the entire policy must be enclosed with parentheses, and each constituent master list must be separated by a comma as shown in the following example:

(MASTER_LIST=(node1,node2), MASTER_LIST=(node3,node4))

There is no significance to the position of a system name in a master list.


The COUNT keyword specifies the number of the systems, which are named in the master list, that can have a master bitmap. Therefore, the COUNT keyword must be associated with a specific master list by enclosing both with parentheses.

A COUNT value of n means that you want master bitmaps on any n systems in the associated master list. It does not necessarily mean that the first n systems in the list are chosen.

The COUNT keyword is optional. When omitted, the default value is the the number of systems in the master list or the value of 6, whichever is less. You cannot specify more than one COUNT keyword for any one master list.

The following two examples are valid policies:

(MASTER_LIST=(node1,node2,node3), COUNT=2)

(MASTER_LIST=(node1,node2,node3),COUNT=2),(COUNT=2, MASTER_LIST=(system4,system5,system6)

In contrast, the following example is not valid because the COUNT keyword is not grouped with a specific master list:

(MASTER_LIST=(node1,node2), MASTER_LIST=(node4,node5), COUNT=1)


The RESET_THRESHOLD keyword specifies the number of blocks that can be set before the bitmap is eligible to be cleared. Each bit that is set in a master bitmap corresponds to a set of blocks that needs to be merged. Therefore, the merge time can be influenced by this value.

Bitmaps are eligible to be cleared when the RESET_THRESHOLD is exceeded. However, the reset is not guaranteed to occur immediately when the threshold is crossed. For additional information about choosing a value for this attribute, see Section 6.8.2 and Section 6.10.2.

A single reset threshold value is associated with any given HBMM policy. Therefore, the RESET_THRESHOLD keyword cannot be specified more than once in a given policy specification. Because its scope is the entire policy, the RESET_THRESHOLD keyword cannot be specified inside a constituent master list when the policy uses multiple master lists.

When the RESET_THRESHOLD keyword is omitted, the value of 50000 is used by default.

The following policy example includes an explicit reset threshold value:


6.7 Rules Governing HBMM Policies

The following rules govern the creation and management of HBMM policies. The rules are based on the assumption that a shadow set is mounted on a system that supports HBMM.

Policies and Their Attributes

  • A policy can be assigned to a shadow set by specifying only its attributes. The number of policies that you can assign in this way is limited only by the number of shadow sets that are supported on a system.
  • A shadow set can have only one HBMM policy associated with it at a time.
  • Policies are in effect clusterwide.
  • Policy names must conform to the following rules:
    • A policy name can be from 1 to 64 characters in length and is case insensitive.
    • Only letters, numbers, the dollar sign ($), and the underscore (_) are allowed.
  • A policy name must be specified in full; abbreviations are not allowed.
  • A named policy can be assigned to a shadow set only by the SET SHADOW/POLICY=HBMM=policy-name command.
  • The limit on user-defined, named policies is 128.


The named policies DEFAULT and NODEFAULT have special properties, as summarized in the following sections:

    • A DEFAULT policy is useful if the majority of the shadow sets in a cluster are expected to use an identical policy.
    • You can create a DEFAULT policy by defining a named policy with the reserved name DEFAULT. No predetermined DEFAULT policy is provided by HP.
    • When a policy with the reserved name of DEFAULT is defined, this policy is associated with a shadow set by any of the following operations:
      • Mount of a shadow set without an associated policy
        The DEFAULT policy, if defined, is applied to a shadow set in the absence of an assigned policy (including the NODEFAULT policy). For example, when shadow set DSA1 is mounted on an HBMM-capable system, an attempt is made to apply an HBMM policy, if one exists, that is specific to DSA1. (To verify whether a device-specific policy exists and to display specific policies, see Section 6.9.9.)
        If a policy has not been defined specifically for DSA1, an attempt is made to apply the DEFAULT policy. If the DEFAULT policy exists, the attributes of that policy are applied to DSA1.
      • End of merge of a shadow set without an associated policy
      • Use of SET SHADOW/ENABLE=HBMM command
    • If a shadow set has a policy association and that policy association is deleted, it is then eligible for the DEFAULT policy, if one was established for your cluster.
  • NODEFAULT Policy
    • The NODEFAULT policy specifies that the shadow set to which it is applied will not use HBMM; no HBMM bitmaps are created anywhere in the cluster for this shadow set.
    • In a cluster where a DEFAULT policy has been defined, the NODEFAULT policy can be used to prevent specific shadow sets from receiving the default policy.
    • The NODEFAULT policy cannot be deleted or redefined.

Assignment and Activation of a Policy

  • A policy can be assigned to a shadow set before the shadow set is mounted on any system in the cluster.
  • If a policy has been assigned, it is activated by the first mount of a shadow set on a system capable of having a master bitmap.
  • Assigning a policy implicitly enables HBMM on a mounted shadow set if it is mounted on a system that can create a master bitmap. Consider DSA1 that is mounted on system MAPLE. When DSA1 was mounted, no HBMM policy was set for DSA1, nor was there a DEFAULT policy that would be applied. Later, the following command is used:


    Because DSA1 is already mounted on system MAPLE, HBMM becomes enabled as a result of the policy assignment (see Section 6.9.2).
  • Any attempt to enable HBMM by means of the SET SHADOW DSAn /ENABLE=HBMM returns a failure if a shadow set is not mounted on a system that has a master bitmap, or if the policy has not been defined.
  • As new systems join the cluster, they inherit the policies in existence in that cluster.

Changes to Policies

  • Named policies can be created, changed, and deleted at will. Changes made to a named policy are not inherited by any mounted shadow set that was assigned the previous version of that named policy.
  • The association of a policy with a mounted shadow set cannot be changed if HBMM is enabled for that shadow set. HBMM must first be disabled on that shadow set, and then a different policy can be assigned to it.
  • Any policy change is clusterwide.

Life of a Policy

  • All policies remain in effect in a cluster as long as at least one system remains active. However, if all systems are shut down, all policy definitions and associations are deleted. The policies must be defined and assigned again when the systems form the cluster. Therefore, HP recommends that you define your desired HBMM policies in your system startup procedures before you mount your shadow sets.
  • Policy assignments persist across the disabling of HBMM or the dismounting of the shadow set as long as at least one system in the cluster remains active.

6.8 Guidelines for Establishing HBMM Policies

Establishing HBMM policies is likely to be an ongoing process as configurations change and as you learn more about how HBMM works and how it affects various operations on your systems. This section describes a number of considerations to help you determine what policies are appropriate for your configuration.

The settings depend on your hardware and software configuration, the computing load, and your operational requirements. These guidelines should assist you with choosing the initial settings for your configuration. As you observe the results in your configuration, you can make further adjustments to suit your computing environment.

6.8.1 Selecting the Systems to Host Master Bitmaps

There are several factors to consider when choosing the number of master bitmaps to specify in a policy and the systems that will host the master bitmaps. The first issue is how many master bitmaps should be used in the configuration. Six is the maximum per shadow set. The use of each additional master bitmap has a slight impact on write performance and also consumes memory on each system (as described in Section 6.5.1).

Using only one master bitmap creates a single point of failure; if the system hosting the master bitmap fails, then this shadow sets undergoes a full merge. Therefore, the memory consumption must be weighed against the adverse effects of a full merge. Using six master bitmaps provides the greatest defense against performing full merges.

Another issue when selecting a system to host the master bitmap is the I/O bandwidth of the various systems. Keep in mind that minimerges are always performed on a system that has a master bitmap. Therefore, low-bandwidth systems, such as satellite cluster members, are not good candidates.

The disaster tolerance of the configuration is also important in the decision process. Specifying systems to host master bitmaps at multiple sites helps ensure that a minimerge is performed if connectivity to an entire site is lost. A two-site configuration should ensure that half the master bitmaps are at each site, and a three-site configuration should ensure that one third of the master bitmaps are at each of the three sites.

6.8.2 Setting the Bitmap RESET_THRESHOLD Value

HBMM bitmaps keep track of writes to a shadow set. The more bits that are set in the bitmap, the greater the amount of merging that is required in the event of a minimerge. HBMM clears the bitmap (after ensuring that all outstanding writes have completed so that the members are consistent) when certain conditions are met (see Section 6.10.2). A freshly cleared bitmap, with few bits set, performs a minimerge much more quickly.

The bitmap reset, however, can be costly to I/O performance. Before a bitmap reset can occur, all write I/O to the shadow set must be paused and any write I/O that is in flight must be completed. Then the bitmap is cleared. This is done on all systems on a per shadow set basis. Therefore, avoid a reset threshold setting that causes frequent resets.

You can view the number of resets performed by using the SHOW SHADOW command, as shown in the following example:

_DSA1031: Volume Label: HBMM1031
  Virtual Unit State:   Steady State
  Enhanced Shadowing Features in use:
        Host-Based Minimerge (HBMM)

  VU Timeout Value      3600    VU Site Value          0
  Copy/Merge Priority   5000    Mini Merge       Enabled
  Served Path Delay     30

  HBMM Policy
    HBMM Reset Threshold: 50000
    HBMM Master lists:
      Up to any 2 of the systems: LEMON, ORANGE
      Any 1 of the systems: MELON, PEACH
    HBMM bitmaps are active on LEMON, MELON, ORANGE
  HBMM Reset Count      76    Last Reset    29-JAN-2004 10:13:53.90
    Modified blocks since last bitmap reset: 40132


Writes that need to set bits in the bitmap are slightly slower than writes to areas that are already marked as having been written. Therefore, if many of the writes to a particular shadow set are concentrated in certain "hot" files, then the reset threshold should be made large enough so that the same bits are not constantly set and then cleared.

On the other hand, if the reset threshold is too large, then the advantages of HBMM are reduced. For example, if 50% of the bitmap is populated (that is, 50% of the shadow set has been written to since the last reset), then the HBMM merge will take approximately 50% of the time of a full merge.

When selecting a threshold reset value, you need to balance the effects of bitmap resets on I/O performance with the time it takes to perform HBMM minimerges. The goal is to set the reset value as low as possible (thus decreasing merge times) while not affecting application I/O performance. Too low a value will degrade I/O performance. Too high a value causes HBMM merges to take extra time.


You can change the reset threshold while a policy is in effect.

6.8.3 Using Multiple Policies

HBMM policies are defined to implement the decisions regarding master bitmaps. Some sites might find that a single policy can effectively implement the decisions. Other sites might need greater granularity and therefore implement multiple policies.

The most likely need for multiple policies is when the cluster includes enough high-bandwidth systems that you want to ensure that the merge load is spread out. Remember, minimerges occur only on systems that host a master bitmap. So, if 12 systems with high bandwidth are set up to perform minimerge or merge operations (the system parameter SHADOW_MAX_COPY is greater than zero on all systems), then you should ensure that the master bitmaps are spread out among these high-bandwidth systems.

Multiple HBMM policies are also useful when shadow sets need different bitmap reset thresholds. The master list can be the same for each policy, but the threshold can differ.

6.9 Configuring and Managing HBMM

This section describes the major tasks for configuring and managing HBMM.

6.9.1 How to Define an HBMM Policy

The SET SHADOW/POLICY=HBMM command is used to define HBMM policies. You can define multiple policies for your environment. The following examples show how to define two policies, a DEFAULT policy and POLICY_1, a named policy.

To define the policy named DEFAULT:


In this example, a DEFAULT policy is created for the cluster. The use of the asterisk wildcard (*) means that any system can host a master bitmap. The omission of the keyword COUNT=n means that up to six systems (the default and the current maximum supported) can host a master bitmap. The DEFAULT policy is inherited at mount time by shadow sets that have not been assigned a named policy.

The following example defines a named policy (POLICY_1), specifies the systems that are eligible to host a master bitmap, limits to two the number of systems that can host a master bitmap, and specifies a higher threshold (default is 50,000 blocks) to be reached before clearing the bitmap.

_$ RESET_THRESHOLD=100000) -

For the full DCL syntax for the SET SHADOW/POLICY=HBMM command, see HP OpenVMS DCL Dictionary.

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