Document revision date: 30 March 2001

## Volume Shadowing for OpenVMS

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## Glossary

An alphabetical list of terms used in Volume Shadowing for OpenVMS and their definitions follow.

assisted copy: An assisted copy is a copy operation performed within an HSx controller in the configuration. The assisted copy does not transfer data through the host node memory. Because the data transfer is from disk to disk, the assisted copy decreases the impact on the system, the I/O bandwidth consumption, and the time required for copy operations. The shadowing software controls the copy operation by using special MSCP copy commands called disk copy data (DCD) commands to instruct the controller to copy specific ranges of logical blocks. For an assisted copy, only one disk can be an active target for a copy at a time.

copy: A copy operation, in the context of Volume Shadowing for OpenVMS, is the process of duplicating the contents of one device onto a second device.

copy fence: A copy fence is a logical boundary between the blocks that have been copied and those that remain to be copied. A copy fence advances with the completion of each copy operation.

DCD: DCD is the acronym for disk copy data, the name of some specialized MSCP commands. The DCD commands are invoked by shadowing software to control assisted copy operations between disks connected to an HSJ controller.

device: Hardware that allows access to storage media; also called drive.

device driver: A software component of the operating system that allows the host computer to communicate with the controller of a device. A device driver exists on the host computer for every peripheral device to which it is attached.

disk: Physical media on which files reside.

dissolve: The act of removing a shadow set from a configuration by removing the virtual unit.

drive: Hardware that allows access to storage media; also called device.

generation number: A generation number is the time stamp assigned to all members of a shadow set by the shadowing software, which the shadowing software uses to track changes in the composition of the shadow set. If a member is removed from a shadow set, the shadowing software updates the generation number of the remaining members.

local write bitmap: A local write bitmap is a bitmap that is created when you mount or dismount a minicopy-enabled shadow set. A local write bitmap communicates with the master write bitmap to ensure that the master write bitmap has a record of all changed blocks. See also write bitmap and master write bitmap.

logical block: Organizational unit of volume space.

logical block number (LBN): A number that identifies a block on a volume. Logical block numbering begins with the first byte in the volume space and continues in a sequentially ascending order through the remainder of the volume space.

master write bitmap: A master write bitmap is created on the first OpenVMS Alpha system that mounts the shadow set. It contains a record of all blocks that have been changed on a shadow set. See also local write bitmap and write bitmap.

merge: A merge operation is an operation to resolve any data inconsistencies between members of a shadow set that could occur when a system fails. A merge operation is declared by the shadowing software for all shadow sets that were mounted on a system that failed.

merge fence: A merge fence is a logical boundary between the blocks that have been compared and those that remain to be compared. A merge fence advances with the completion of each comparison.

minicopy: A minicopy operation is similar to a copy operation, as defined in the context of Volume Shadowing for OpenVMS, except that it copies only the changed blocks. Therefore, the time to perform a minicopy is proportional to the amount of changed blocks on the device. A minicopy operation relies on the existence of a write bitmap for the shadow set.

minimerge: A minimerge operation is similar to a merge operation but faster and requires an HSx controller in the configuration. The shadowing software uses a controller-based write log, provided by the HSx controller, which shows exactly which blocks had write I/O requests and data security erases (DSEs) outstanding. Only these blocks are made identical.

shadow set: A shadow set consists of up to three devices that are logically bound together by Volume Shadowing for OpenVMS software. The shadow set members are assigned the same virtual unit number, which is stored in the device's storage control block (SCB).

shadow set member: A shadow set member is a device that has been logically bound with other devices into a shadow set.

source device : The device whose contents are copied to a target device.

System Communications Services (SCS): In an OpenVMS Cluster environment, software that implements intercomputer communication, according to the System Communications Architecture (SCA).

target: The device to which the contents of a shadow set member is being copied. When the copy is complete, the target is a member of the shadow set.

virtual unit: A shadow set is represented as a single virtual device, called a virtual unit. A virtual unit is identified by its name DSAn, where n can be any number between 0 and 9999.

volume: Disk or tape media that has been prepared for use by creating a new file structure on it and mounting it on a device.

volume set: A collection of disk volumes bound into a single entity by the DCL command MOUNT/BIND. To users, a volume set looks like a single, large volume.

Also, the volumes on which a set of multivolume files is recorded.

write bitmap: A write bitmap is a data structure in memory that tracks the addresses of all write operations and all data security erase (DSE) operations. See also master write bitmap and local write bitmap.

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