> But the whole point of snapshots is that they don't > take up extra space on the disk. If a file (and > hence a block) is in every snapshot it doesn't mean > you've got multiple copies of it. You only have one > copy of that block, it's just referenced by many > snapshots.
I used the wording "copies of a parent" loosely to mean "previous states of the parent that also contain pointers to the current state of the child about to be updated in place". > > The thing is, the location of that block isn't saved > separately in every snapshot either - the location is > just stored in it's parent. And in every earlier version of the parent that was updated for some *other* reason and still contains a pointer to the current child that someone using that snapshot must be able to follow correctly. So moving a block is > just a case of updating one parent. No: every version of the parent that points to the current version of the child must be updated. ... > If you think about it, that has to work for the old > data since as I said before, ZFS already has this > functionality. If ZFS detects a bad block, it moves > it to a new location on disk. If it can already do > that without affecting any of the existing snapshots, > so there's no reason to think we couldn't use the > same code for a different purpose. Only if it works the way you think it works, rather than, say, by using a look-aside list of moved blocks (there shouldn't be that many of them), or by just leaving the bad block in the snapshot (if it's mirrored or parity-protected, it'll still be usable there unless a second failure occurs; if not, then it was lost anyway). - bill This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
