FlashCopy

FlashCopy is an IBM feature supported on various IBM storage devices that made it possible to create, nearly instantaneously, point-in-time snapshot copies of entire logical volumes or data sets.^[1] The Hitachi Data Systems implementation providing similar function was branded as ShadowImage. Using either implementation, the copies are immediately available for both read and write access.

Implementations

Version 1

The first implementation of FlashCopy, Version 1 allowed entire volumes to be instantaneously “copied” to another volume by using the facilities of the newer Enterprise Storage Subsystems (ESS).^[2]

Version 1 of FlashCopy had limitations however. Although the copy or “flash” of a volume occurred instantaneously, the FlashCopy commands were issued sequentially and the ESS required a brief moment to establish the new pointers. Because of this minute processing delay, the data residing on two volumes that were FlashCopied are not exactly time consistent.^[2]

Version 2

FlashCopy Version 2 introduced the ability to flash individual data sets and then added support for “consistency groups”. FlashCopy consistency groups can be used to help create a consistent point-in-time copy across multiple volumes, and even across multiple ESSs, thus managing the consistency of dependent writes.^[2]

FlashCopy consistency groups are used in a single-site scenario in order to create a time-consistent copy of data that can then be backed-up and sent off site, or in a multi-site Global Mirror for ESS implementation to force time consistency at the remote site.^[2]

The implementation of consistency groups is not limited to FlashCopy. Global Mirror for IBM System z series (formerly known as XRC or eXtended Remote Copy) also creates consistency groups to asynchronously mirror disk data from one site to another over any distance .^[2]

References