Okay, since I'm finally starting to care (projecting long-term use, so I can set my snapshot taking and retention policy, and estimate disk needs), I need a review of reading snapshot size information.
>From the user end, there are three "sizes" that I can imagine caring about for a snapshot: 1) The total amount of space that sending this snapshot to a new filesystem would take. This is the largest possible value for snapshot "size" -- the size of everything that can be accessed through this snapshot. I believe this is what zfs list reports as "referenced". 2) The amount of space that would be freed up in the pool if this snapshot were to be destroyed. This has the obvious use -- if you're short of space and looking for something to delete, this is the number you need to consider. I believe this is "usedbydataset". 3) The amount of space needed to represent the difference between this snapshot and the preceding stored state. This should be roughly the size that an incremental ZFS send would be from the preceding state to this state. (2 and 3 would be the same IF the snapshot in question was the most recent state of the filesystem, nothing changed since then.) The "used" space (zfs list output) isn't any of these; it's what the snapshot plus all descendents uses. And the "usedby*" give lots of other kinds of detail. "used" is changes in this and later datasets, I guess. So the difference between "used" in adjacent datasets is my #3, at least roughly. (And I understand that there are timing issues involved in testing expecting to see exact numbers.) I guess this stuff is decently documented; at least unless I misunderstood a bunch. Let me know if anything is badly wrong! -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
