> It'd be nice if the DASD boxes could have a "copy on write" feature > akin to > the Linux memory manager; This kind of technology would be > reasonable for > handling things like R/O images. Or am I confusing this with GPFS?
No, that's essentially how the feature works. You ask for a point-in-time copy of a DASD image - the controller says 'Done' immediately and you can start using it. The controller starts a real copy operation in the background. If you read a page that hasn't been copied yet, you get the original. If you read one that's been copied, you get the copy. If you write one, it's written where it should go in the copy and the controller makes a note not to overwrite it with the original when it gets around to that page. Various brand names - Storage Technology was first with 'Snapshot'. -- Phil Payne http://www.isham-research.com +44 7785 302 803
