Patent ID: 7509523

Claim:
A storage controller comprising an interface to a host computer apparatus and an interface to a plurality of controlled storage apparatus, and comprising: a write component operable to send a request to write a data object to a source data image at one of said plurality of controlled storage apparatus; a copy control component operable in response to a metadata state to control copying of said data object to a target data image in a delimited sequence of data images in a cascade at said plurality of controlled storage apparatus; and a substitution component operable in response to a detection of an offline condition of a disk (A) containing said target data image to control substitution of a fill-in disk (F) in place of said disk containing said target data image; said substitution component further operable, in response to detection of the offline condition of A, to impose a first rule on subsequent writes to A that have resulted from writes to other disks, the first rule governing which bitmaps to use in order to decide the state of a grain and being expressible as: if grain on F is Blank Data then use bitmaps for A else use bitmaps for F record the effect of the write (if any) in the bitmaps of F and to impose a second rule on subsequent writes to A expressible as: if A is inaccessible then all writes to A are failed else all writes are redirected to F and the bitmaps of F are changed to record the effect of the write.