"Ferd Berfl" ferd_berfl@yahoo.com writes:
re: snapshots, I thought that everyone else's snapshot functionality other than Netapp's must be based on a "copy on write" method because Netapp has a patent on continuously writing new blocks. (their WAFL filesystem intellectual property and patents). If you can't take advantage of the method of writing modified blocks as new blocks and not touching the original block pointed to by the snapshot, doesn't that mean that you have to reserve some amount of storage, that is free blocks, for holding the copy on write changed blocks to maintain the snapshots?
I am not sure what the NetApp patents claim, but if they claim to have originated the idea of always writing an updated block in a new location, that's an old idea with plenty of prior art.
For example, EDF minidisks under VM/CMS had exactly that idea, including the concept of consistency points. (It wouldn't have been all that big a stretch to have invented snapshots for them, actually.) Of course that was in the context of a mostly-single-user filing system.
In the context of database implementations rather than general-purpose filing systems, I can recall these ideas being used in the early 1970s.
Chris Thompson University of Cambridge Computing Service, Email: cet1@ucs.cam.ac.uk New Museums Site, Cambridge CB2 3QG, Phone: +44 1223 334715 United Kingdom.