skip book previous and next navigation links
go up to top of book: HP Volume Shadowing for OpenVMS HP Volume Shadowing for OpenVMS
go to beginning of chapter: Ensuring Shadow Set Consistency Ensuring Shadow Set Consistency
 
go to next page: Copy OperationsCopy Operations
end of book navigation links

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:

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 will differ 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:

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

Information in the Storage Control Block (SCB) lists some of the information contained in the SCB.

Table 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 will occur 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.


 
go to next page: Copy OperationsCopy Operations