Document revision date: 15 July 2002

The purpose of a merge operation is to compare data on shadow set members and to ensure that inconsistencies are resolved. A merge operation is initiated if either of the following events occurs:

• A system failure results in the possibility of incomplete writes.
  For example, if a write request is made to a shadow set but the system fails before a completion status is returned from all the shadow set members, it is possible that:
  • All members might contain the new data.
  • All members might contain the old data.
  • Some members might contain new data and others might contain old data.
  
  The exact timing of the failure during the original write request defines which of these three scenarios results. When the system recovers, however, it is essential that corresponding LBNs on each shadow set member contain the same data (old or new). Thus, the issue here is not one of data availability, but rather of reconciling potential differences among shadow set members. Once the data on all disks is made identical, application data can be reconciled, if necessary, either by the user reentering the data or by database recovery and application journaling techniques.
• If a shadow set enters mount verification with outstanding write I/O in the internal queue, and the problem is not corrected before mount verification times out, the systems on which the timeout occurred cause the shadow set to require a merge operation.
  For example, if the shadow set were mounted on eight nodes and mount verification timed out on two of them, those two nodes would each cause the shadow set to require a merge operation.

The merge operation is managed by one of the OpenVMS systems that has the shadow set mounted. The members of a shadow set are physically compared to each other to ensure that they contain the same data. This is done by performing a block-by-block comparison of the entire volume. As the merge proceeds, any blocks that are different are made the same --either both old or new---by means of a copy operation. Because the shadowing software does not know which member contains newer data, any full member can be the source member of the merge operation.

The shadowing software always selects one member as a logical master for any merge operation, across the OpenVMS Cluster. Any difference in data is resolved by a propagation of the information from the merge master to all the other members.

The system responsible for doing the merge operation on a given shadow set, updates the merge fence for this shadow set after a range of LBNs is reconciled. This fence "proceeds" across the disk and separates the merged and unmerged portions of the shadow set.

Application read I/O requests to the merged side of the fence can be satisfied by any source member of the shadow set. Application read I/O requests to the unmerged side of the fence are also satisfied by any source member of the shadow set; however, any potential data differences---discovered by doing a data compare operation---are corrected on all members of the shadow set before returning the data to the user or application that requested it.

This method of dynamic correction of data inconsistencies during read requests allows a shadow set member to fail at any point during the merge operation without impacting data availability.

Volume Shadowing for OpenVMS supports both assisted and unassisted merge operations in the same cluster. Whenever you create a shadow set, add members to an existing shadow set, or boot a system, the shadowing software reevaluates each device in the changed configuration to determine whether it is capable of supporting the merge assist.

6.3.1 Unassisted Merge Operations

For systems running software earlier than OpenVMS Version 5.5--2, the merge operation is performed by the system and is known as an unassisted merge operation.

To ensure minimal impact on user I/O requests, volume shadowing implements a mechanism that causes the merge operation to give priority to user and application I/O requests.

The shadow server process performs merge operations as a background process, ensuring that when failures occur, they minimally impact user I/O. A side effect of this is that unassisted merge operations can often take an extended period of time to complete, depending on user I/O rates. Also, if another node fails before a merge completes, the current merge is abandoned and a new one is initiated from the beginning.

Note that data availability and integrity are fully preserved during merge operations regardless of their duration. All shadow set members contain equally valid data.

6.3.2 Assisted Merge Operations

Starting with OpenVMS Version 5.5--2, the merge operation includes enhancements for shadow set members that are configured on controllers that implement assisted merge capabilities. The assisted merge operation is also referred to as a minimerge. The minimerge feature significantly reduces the amount of time needed to perform merge operations. Usually, the minimerge completes in a few minutes.

HSC and HSJ controllers support minimerge. Support for minimerge on HSG controllers is planned.

By using information about write operations that were logged in controller memory, the minimerge is able to merge only those areas of the shadow set where write activity was known to have been in progress. This avoids the need for the total read and compare scans required by unassisted merge operations, thus reducing consumption of system I/O resources.

Controller-based write logs contain information about exactly which LBNs in the shadow set had write I/O requests outstanding (from a failed node). The node that performs the assisted merge operation uses the write logs to merge those LBNs that may be inconsistent across the shadow set. No controller-based write logs are maintained for a one member shadow set. No controller-based write logs are maintained if only one OpenVMS system has the shadow set mounted.

The minimerge operation is enabled on nodes running OpenVMS Version 5.5--2 or later. Volume shadowing automatically enables the minimerge if the controllers involved in accessing the physical members of the shadow set support it. See the Volume Shadowing for OpenVMS Software Product Description (SPD 27.29.xx) for a list of supported controllers. Note that minimerge operations are possible even when shadow set members are connected to different controllers. This is because write log entries are maintained on a per controller basis for each shadow set member.

Volume Shadowing for OpenVMS automatically disables minimerges if:

• The shadow set is mounted on a cluster node that is running an OpenVMS release earlier than Version 5.5--2.
• A shadow set member is mounted on a controller running a version of firmware that does not support minimerge.
• A shadow set member is mounted on a controller that has performance assists disabled.
• If any node in the cluster, with a shadow set mounted, is running a version of Volume Shadowing that has minimerge disabled.
• The shadow set is mounted on a standalone system. (Minimerge operations are not enabled on standalone systems.)
• The shadow set is mounted on only one node in the OpenVMS Cluster.

The following transient conditions can also cause a minimerge operation to be disabled:

• If an unassisted merge operation is already in progress when a node fails.
  In this situation, the shadowing software cannot interrupt the unassisted merge operation with a minimerge.
• When not enough write log entries are available in the controllers.
  The number of write log entries available is determined by controller capacity. The shadowing software dynamically determines when there are enough entries to maintain write I/O information successfully. If the number of available write log entries is too low, shadowing temporarily disables logging for that shadow set, and it returns existing available entries on this and every node in the cluster. After some time has passed, shadowing will attempt to reenable write logging on this shadow set.
  A controller retains a write log entry for each write I/O request until that entry is deleted by shadowing, or the controller is restarted.
  A multiple-unit controller shares its write log entries among multiple disks. This pool of write log entries is managed by the shadowing software. If a controller runs out of write log entries, shadowing disables minimerges and will perform an unassisted merge operation, should a node leave the cluster without first dismounting the shadow set. Note that write log exhaustion does not typically occur with disks on which the write logs are not shared.
• When the controller write logs become inaccessible for one of the following reasons, a minimerge operation is not possible.
  • Controller failure causes write logs to be lost or deleted.
  • A device that is dual ported to multiple controllers fails over to its secondary controller. (If the secondary controller is capable of maintaining write logs, the minimerge operations are reestablished quickly.)

This section describes how to control assisted copy and minimerge operations on an HSC controller. It is not possible to control these operations on an HSJ controller.

To disable both the merge and copy performance assists on the HSC controller, follow these steps on each HSC controller for which you want to disable the assists:

1. Press Ctrl/C to get to the HSC prompt.
2. When the HSC> prompt appears on the terminal screen, enter the following commands:

   HSC> RUN SETSHO SETSHO> SET SERVER DISK/NOHOST_BASED_SHADOWING SETSHO-I Your settings require an IMMEDIATE reboot on exit. SETSHO> EXIT SETSHO-Q Rebooting HSC. Press RETURN to continue, CTRL/Y to abort:

After you issue these commands, the HSC controller automatically reboots:

INIPIO-I Booting...

To reenable the assists, follow the same procedure on your HSC controller, but use the /HOST_BASED_SHADOWING qualifier on the SET SERVER DISK command.

Use the HSC command SHOW ALL to see whether the assists are enabled or disabled. The following example shows a portion of the SHOW ALL display that indicates the shadowing assists status:

HSC> SHOW ALL . . . 5-Jun-1997 16:42:51.40 Boot: 21-Feb-1997 13:07:19.47 Up: 2490:26 Version: V860 System ID: %X000011708247 Name: HSJNOT Front Panel: Secure HSC Type: HSC90 . . . Disk Server Options: Disk Caching: Disabled Host Based Shadowing Assists: Enabled Variant Protocol: Enabled Disk Drive Controller Timeout: 2 seconds Maximum Sectors per Track: 74 sectors Disk Copy Data connection limit: 4 Active: 0 . . .

6.5 What Happens to a Shadow Set When a System Fails?

When a system, controller, or disk failure occurs, the shadowing software maintains data availability by performing the appropriate copy, merge, or minimerge operation. The following subsections describe the courses of action taken when failures occur. The course of action taken depends on the event and whether the shadow set is in a steady state or a transient state.

Transitions from Steady State

When a shadow set is in a steady state, the following transitions can occur:

• If you mount a new disk into a steady state shadow set, the shadowing software performs a copy operation to make the new disk a full shadow set source member.
• If a failure occurs on a standalone system (the system crashes), on a steady state shadow set, the shadow set SCB reflects that the shadow set has been incorrectly dismounted. When the system is rebooted and the set is remounted, a copy operation is not necessary, but a merge operation is necessary and initiated.
• If a failure occurs in a cluster, the shadow set is merged by a remaining node that has the shadow set mounted:
  • If performance assists are enabled, and the controller-based write logs are available, the shadowing software performs a minimerge.
  • If performance assists are not enabled, the shadowing software performs a merge operation.

Once the transition completes, the disks contain identical information and the shadow set returns to a steady state.

Transitions During Copy and Minicopy Operations

The following list describes the transitions that can occur to a shadow set that is undergoing a copy or minicopy operation. The transitions apply to both forms of copy operations except where noted:

• If you mount an additional disk into the shadow set that is already undergoing a copy operation, the shadowing software finishes the original copy operation before it begins another copy operation on the newly mounted disk.
• When a shadow set on a standalone system is undergoing a copy operation and the system fails, the copy operation aborts and the shadow set is left with the original members. For a standalone system, there is no recourse except to reboot the system and reinitiate the shadow set copy operation with a MOUNT command.
• When a shadow set is mounted on more than one node in the cluster and is undergoing a copy operation, if the node performing the copy operation dismounts the virtual unit, another node in the cluster that has that shadow set mounted will continue the copy operation automatically.
  If a shadow set is undergoing a minicopy operation when this occurs, the minicopy will not continue. Instead, a full copy will continue from the point where the minicopy stopped, and all the remaining blocks will be copied.
• If a shadow set is mounted on more than one node in the cluster and is undergoing a copy operation, should the node performing the copy operation fail, another node in the cluster that has that shadow set mounted will continue the copy operation automatically.

When a node failure occurs during a shadow set copy operation, merge behavior depends on whether or not the shadowing performance assists are enabled.

• If minimerge is enabled and can be performed, the shadowing software interrupts the copy operation to perform a minimerge and then resumes the copy operation.
• If the minimerge is not enabled, the shadowing software marks the set as needing a merge operation and finishes the copy operation before beginning the merge operation.

Transitions During Minimerge Operations

When a shadow set is undergoing a minimerge operation, the following transitions can occur:

• If a new member is mounted into a shadow set when a minimerge operation is in progress, the minimerge is completed before the copy operation is started.
• If another system failure occurs before a pending minimerge has completed, the action taken depends on whether or not the shadowing performance assists are enabled and if the controller-based write logs are available.
  • If performance assists are enabled and if the controller-based write logs are available for the last node failure, the shadowing software restarts the minimerge from the beginning and adds new LBNs to the write log file based on the entries taken from the nodes that failed.
  • If performance assists are disabled, the shadowing software reverts to a merge operation. The performance assists might be disabled if the controller runs out of write logs or if a failover occurs from a controller with write logs to one that does not.

Transitions During Merge Operations

The following list describes the transitions that can occur to the shadow set that is undergoing a merge operation when performance assists are not available:

• If you add a new disk to a shadow set that is undergoing a merge operation, the shadowing software interrupts the merge operation to perform a copy operation. The merge operation resumes when the copy operation is completed.
• If a node failure occurs when the shadow set is performing a merge operation, the shadowing software abandons the current merge operation and starts a new merge operation.

Example 6-1 shows what happens when you create a shadow set by mounting two disk volumes that have never been a part of a shadow set. Because neither disk volume has been a part of a shadow set, the Mount utility (MOUNT) assumes that the first disk named in the MOUNT command is the source member. When the Mount utility checks the volume labels on the disks, it discovers that they are different from each other, and the utility automatically performs a copy operation.

In this example, DSA0 is the virtual unit name, $1$DUA8 and $1$DUA89 are the names of the disk volumes, and SHADOWDISK is the volume label.

Example 6-1 Copy Operation: Creating a New Shadow Set

$ MOUNT DSA0: /SHADOW=($1$DUA8:,$1$DUA89:) SHADOWDISK %MOUNT-I-MOUNTED, SHADOWDISK mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA8: (FUSS) is now a valid member of the shadow set %MOUNT-I-SHDWMEMCOPY, _$1$DUA89: (FUSS) added to the shadow set with a copy operation $ SHOW DEVICE DSA0: Device Device Error Volume Free Trans Mnt Name Status Count Label Blocks Count Cnt DSA0: Mounted 0 SHADOWDISK 890937 1 1 $1$DUA8: (FUSS) ShadowSetMember 0 (member of DSA0:) $1$DUA89: (FUSS) ShadowCopying 0 (copy trgt DSA0: 1% copied)

The SHOW DEVICE display in Example 6-1 shows the shadow set during the copy operation (transient state). Because the SCB information on $1$DUA8 and $1$DUA89 indicates that these devices have never been part of a shadow set, the shadowing software uses the first device named in the command line ($1$DUA8) as the source of the copy operation. The device status "ShadowSetMember" indicates that the $1$DUA8 device is a source shadow set member, and "ShadowCopying" indicates that the physical device $11$DUA89 is the target of a copy operation.

Suppose you want to add a new member to an existing shadow set, and the device you add is a previous member of this same shadow set. In this case, the volume label of the new member matches that of the current shadow set members, but the new member's MOUNT generation number is out of date compared with those of the current members. Thus, the Mount utility automatically performs a copy operation on that member.

Example 6-2 shows the format of the MOUNT command and MOUNT status messages returned when you add the $3$DIA12 device to the shadow set represented by the DSA9999 virtual unit. Notice that you do not need to list the member units currently in the shadow set on the MOUNT command line.

Example 6-2 Copy Operation: Adding a Member to an Existing Shadow Set

$ MOUNT /SYSTEM DSA9999: /SHADOW=$3$DIA12: AXP_SYS_071 %MOUNT-I-MOUNTED, AXP_SYS_071 mounted on _DSA9999: %MOUNT-I-SHDWMEMCOPY, _$3$DIA12: (SHAD03) added to the shadow set with a copy operation $ SHOW DEVICE DSA9999: Device Device Error Volume Free Trans Mnt Name Status Count Label Blocks Count Cnt DSA9999: Mounted 0 AXP_SYS_071 70610 1 1 $3$DIA7: (BGFUSS) ShadowSetMember 0 (member of DSA9999:) $3$DIA5: (SHAD03) ShadowSetMember 0 (member of DSA9999:) $3$DIA12: (SHAD03) ShadowCopying 0 (copy trgt DSA9999: 0% copied)

Example 6-3 shows what happens when a three-member shadow set is dissolved on one node and then is immediately remounted on another node. When the Mount utility checks the volume information on each member, it finds that the volume information is consistent across the shadow set. Thus, a copy operation is not necessary when the shadow set is mounted.

In Example 6-3, DSA10 is the virtual unit and $3$DUA10, $3$DUA11, and $3$DUA12 are the member volumes. The first part of the example displays the output from a SHOW DEVICE command, which shows that the shadow set is mounted and in a steady state. Then the user dismounts the DSA10 shadow set and immediately remounts it.

Example 6-3 No Copy Operation: Rebuilding a Shadow Set

$ SHOW DEVICE D Device Device Error Volume Free Trans Mnt Name Status Count Label Blocks Count Cnt DSA10: Mounted 0 VAX_SYS_071 292971 1 1 $3$DUA10: (MYNODE) ShadowSetMember 0 (member of DSA10:) $3$DUA11: (MYNODE) ShadowSetMember 0 (member of DSA10:) $3$DUA12: (MYNODE) ShadowSetMember 0 (member of DSA10:) $ DISMOUNT /NOUNLOAD DSA10: %%%%%%%%%%% OPCOM 24-MAR-1997 20:26:41.40 %%%%%%%%%%% $3$DUA10: (MYNODE) has been removed from shadow set. %%%%%%%%%%% OPCOM 24-MAR-1997 20:26:41.69 %%%%%%%%%%% $3$DUA11: (MYNODE) has been removed from shadow set. %%%%%%%%%%% OPCOM 24-MAR-1997 20:26:41.69 %%%%%%%%%%% $3$DUA12: (MYNODE) has been removed from shadow set. %%%%%%%%%%% OPCOM 24-MAR-1997 20:26:41.69 %%%%%%%%%%% $ MOUNT /SYSTEM DSA10: /SHADOW=($3$DUA10:, $3$DUA11:, $3$DUA12:) VAX_SYS_071 %MOUNT-I-MOUNTED, VAX_SYS_071 mounted on _DSA10: %MOUNT-I-SHDWMEMSUCC, _$3$DUA10: (MYNODE) is now a valid member of the shadow set %MOUNT-I-SHDWMEMSUCC, _$3$DUA11: (MYNODE) is now a valid member of the shadow set %MOUNT-I-SHDWMEMSUCC, _$3$DUA12: (MYNODE) is now a valid member of the shadow set $

Example 6-4 shows the output from the SHOW DEVICE command at the time of the merge operation.

When a system fails, the volume information is left in a state that shows that each shadow set member was not properly dismounted. If you issue the MOUNT command again after the node reboots, the shadowing software automatically performs a merge operation on the shadow set.

Example 6-4 Merge Operation: Rebuilding a Shadow Set

$ SHOW DEVICE DSA42: Device Device Error Volume Free Trans Mnt Name Status Count Label Blocks Count Cnt DSA42: Mounted 0 ATHRUZ 565997 1 1 $4$DUA2: (MYNODE) ShadowMergeMbr 0 (merging DSA42: 0% merged) $4$DUA42: (YRNODE) ShadowMergeMbr 0 (merging DSA42: 0% merged)

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