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 HP Volume Shadowing for OpenVMS |
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assisted copy An assisted copy is a copy operation performed withinan HSC or HSJ controller in the configuration. The assisted copydoes not transfer data through the host node memory. Because thedata transfer is from disk to disk, the assisted copy decreasesthe impact on the system, the I/O bandwidth consumption, and thetime required for copy operations. The shadowing software controlsthe copy operation by using special MSCP copy commands called diskcopy data (DCD) commands to instruct the controller to copy specificranges of logical blocks. For an assisted copy, only one disk canbe an active target for a copy at a time.
copy A copy operation, in the context of Volume Shadowingfor OpenVMS, is the process of duplicating the contents of one deviceonto a second device.
copy fence A copy fence is a logical boundary between the blocksthat have been copied and those that remain to be copied. A copyfence advances with the completion of each copy operation.
DCD The acronym for disk copy data, the name of somespecialized MSCP commands. The DCD commands are invoked by shadowingsoftware to control assisted copy operations between disks connectedto an HSJ controller.
device Hardware that allows access to storage media; alsocalled drive.
device driver A software component of the operating system thatallows the host computer to communicate with the controller of adevice. A device driver exists on the host computer for every peripheraldevice to which it is attached.
disk Physical media on which files reside.
dissolve The act of removing a shadow set from a configurationby removing the virtual unit.
drive Hardware that allows access to storage media; alsocalled device.
generation number A generation number is the time stamp assigned toall members of a shadow set by the shadowing software, which theshadowing software uses to track changes in the composition of theshadow set. If a member is removed from a shadow set, the shadowingsoftware updates the generation number of the remaining members..
local write bitmap A local write bitmap is a bitmap that is createdwhen you mount or dismount a minicopy-enabled shadow set. A localwrite bitmap communicates with the master write bitmap to ensure thatthe master write bitmap has a record of all changed blocks. Seealso 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. Logicalblock numbering begins with the first byte in the volume space andcontinues in a sequentially ascending order through the remainderof the volume space.
master write bitmap A master write bitmap is created on the first OpenVMSAlpha system that mounts the shadow set. It contains a record ofall blocks that have been changed on a shadow set. See also localwrite bitmap and write bitmap.
merge A merge operation is an operation to resolve anydata inconsistencies between members of a shadow set that couldoccur when a system fails. A merge operation is declared by theshadowing software for all shadow sets that were mounted on a systemthat failed.
merge fence A merge fence is a logical boundary between theblocks 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, exceptthat it copies only the changed blocks. Therefore, the time to performa minicopy is proportional to the amount of changed blocks on thedevice. A minicopy operation relies on the existence of a writebitmap for the shadow set.
minimerge A minimerge operation is similar to a merge operationbut faster and requires an HSC or HSJ controller in the configuration.The shadowing software uses a controller-based write log, whichshows exactly which blocks had write I/O requests and data securityerases (DSEs) outstanding. Only these blocks are made identical.
shadow set A shadow set consists of up to three devices thatare logically bound together by Volume Shadowing for OpenVMS software.The shadow set members are assigned the same virtual unit number, whichis stored in the device's storage control block (SCB).
shadow set member A shadow set member is a device that has been logicallybound with other devices into a shadow set.
source device The device whose contents are copied to a targetdevice.
System Communications Services (SCS) In an OpenVMS Cluster environment, software that implementsintercomputer communication, according to the System CommunicationsArchitecture (SCA).
target The device to which the contents of a shadow setmember is being copied. When the copy is complete, the target isa member of the shadow set.
virtual unit A shadow set is represented as a single virtualdevice, called a virtual unit. A virtual unit is identified by itsname DSAn, where n can be any number between 0 and 9999.
volume Disk or tape media that has been prepared for useby creating a new file structure on it and mounting it on a device.
volume set A collection of disk volumes bound into a singleentity by the DCL command MOUNT/BIND. To users, a volume set lookslike a single, large volume. Also, the volumes on which a set ofmultivolume files is recorded.
write bitmap A write bitmap is a data structure in memory thattracks the addresses of all write operations and all data securityerase (DSE) operations. See also master write bitmap and local writebitmap.
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