Shadow Set Consistency

During the life of a shadow set, the state of any shadow set member relative to the rest of the members of the shadow set can vary. The shadow set is considered to be in a steady state when all of its members are known to contain identical data. Changes in the composition of the shadow set are inevitable because:

Disk drives occasionally need corrective maintenance.
New disks are added to replace other disks.
System failures occur, requiring merge operations to take place within the shadow set.
Controllers fail, requiring maintenance.
System management functions, such as backup, are required.

For example, suppose an operator dismounts a member of a shadow set and then remounts the member back into the shadow set. During the member's absence, the remaining members of the shadow set may have experienced write operations. Thus, the information on the member being remounted into the shadow set differs from the information on the rest of the shadow set. Therefore, a copy (or minicopy) operation is required.

As another example, consider a situation where a shadow set is mounted by several systems in an OpenVMS Cluster configuration. If one of those systems fails, the data on the members of the shadow set may differ because of outstanding or incomplete write operations issued by the failed system. The shadowing software resolves this situation by performing a merge operation.

In any event, copy and merge operations allow volume shadowing to preserve the consistency of the data written to the shadow set. A shadow set is considered to be in a transient state when one or more of its members are undergoing a copy or a merge operation.

Additionally, volume shadowing maintains shadow set consistency by:

Maintaining consistent data on shadow set members by automatically detecting and replacing bad blocks on one shadow set member and rewriting those bad blocks with good data from another shadow set member.
Notifying all nodes when a member is added or removed from a shadow set, and ensuring the shadow set membership is consistent clusterwide.

Volume shadowing uses two internal mechanisms to coordinate shadow set consistency:

Storage control blocks (SCBs)
Volume shadowing uses a storage control block (SCB) as a primary method for controlling shadow set membership. Each physical disk contains an SCB in which the shadowing software records the names of all the current members of the shadow set. Each time the composition of the shadow set changes, the SCB on all members is updated. This feature simplifies clusterwide membership coordination and is also used by the MOUNT qualifier /INCLUDE to reconstruct a shadow set.
Shadow set generation number
Volume shadowing uses a shadow set generation number as a primary method of determining shadow set member validity and status. A shadow set generation number is an incrementing value that is stored on every member of a shadow set. Each time a membership change occurs to the shadow set (members are mounted, dismounted, or fail), the generation number on the remaining members is incremented. Thus, if a shadow set's generation number is 100 and a member is dismounted from the set, the generation numbers on the remaining members are incremented to 101. The removed member's generation number remains at 100. When mounting shadow sets, the shadowing software uses the generation numbers, found in the SCB on the physical units, to determine the need for and direction of copy operations.

Table 6-1 lists some of the information contained in the SCB.

Table 6-1 Information in the Storage Control Block (SCB)

SCB Information	Function
Volume label	Identifies a unique name for the volume. Every member of a shadow set must use the same volume label.
BACKUP revision number	A BACKUP/IMAGE restoration rearranges the location of data on a volume and sets a revision number to record this change. The Mount utility (MOUNT) checks the revision number of the proposed shadow set member against the numbers on current or other proposed shadow set members. If the revision number differs, the shadowing software determines whether a copy or merge operation is required to bring the data on the less current members up to date.
Volume shadowing generation number	When a member joins a shadow set, it is marked with a volume shadowing generation number. You can zero the generation number by using the /OVERRIDE=SHADOW_MEMBERSHIP qualifier with the MOUNT command.
Mount and dismount status	The SCB mount status field is used as a flag that is set when a volume is mounted and cleared when it is dismounted. There is also a count of the number of nodes that have mounted the shadow set write-enabled. The MOUNT command checks this field when a disk is mounted. If the flag is set, this indicates that the disk volume was incorrectly dismounted. This occurs in the event of system failure. When mounting shadow sets that were incorrectly dismounted, or where the write count field is not correct, the shadowing software automatically initiates merge operations.

Upon receiving a command to mount a shadow set, the volume shadowing software immediately determines whether a copy or a merge operation is required; if either is required, the software automatically performs the operation to reconcile data differences. If you are not sure which disks might be targets of copy operations, you can specify the /CONFIRM or /NOCOPY qualifiers when you use the MOUNT command. To disable performing any copy operations, use the /NOCOPY qualifier. If you mount a shadow set interactively, use the /CONFIRM qualifier to instruct MOUNT to display the targets of copy operations and request permission before the operations are performed.

When you dismount an individual shadow set member, you produce a situation similar to a hardware disk failure. Because files remain open on the virtual unit, the removed physical unit is marked as not being properly dismounted.

After one of the devices is removed from a shadow set, the remaining shadow set members have their generation number incremented, identifying them as being more current than the former shadow set member. This generation number aids in determining the correct copy operation if you remount the member into a shadow set.