On Sat, 19 Dec 2009, Colin Raven wrote:
There is no original, there is no copy. There is one block with reference counters. - Fred can rm his "file" (because clearly it isn't a file, it's a filename and that's all) - result: the reference count is decremented by one - the data remains on disk.
While the similarity to hard links is a good analogy, there really is a unique "file" in this case. If Fred does a 'rm' on the file then the reference count on all the file blocks is reduced by one, and the block is freed if the reference count goes to zero. Behavior is similar to the case where a snapshot references the file block. If Janet updates a block in the file, then that updated block becomes unique to her "copy" of the file (and the reference count on the original is reduced by one) and it remains unique unless it happens to match a block in some other existing file (or snapshot of a file).
When we are children, we are told that sharing is good. In the case or references, sharing is usually good, but if there is a huge amount of sharing, then it can take longer to delete a set of files since the mutual references create a "hot spot" which must be updated sequentially. Files are usually created slowly so we don't notice much impact from this sharing, but we expect (hope) that files will be deleted almost instantaneously.
Bob -- Bob Friesenhahn bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
