As I understand it, the used space of a snapshot does not include anything that is in more than one snapshot. There is a bit of a hack, using the verbose and dry run options of zfs send, that will tell you how much data must be transferred to replicate each snapshot incrementally, which should help clear things up. Try (with elevated privileges):
zfs send -nvR rootpool/export/home@today You might also look at the "REFER" column in "zfs list -r -t snapshot rootpool/export/home", which tells you how much data was referenced by the filesystem when the snapshot was taken. However, this could be uninformative if the majority of the snapshot data is rewritten files rather than new files. Tim On Wed, Aug 29, 2012 at 1:12 PM, Truhn, Chad <chad.tr...@bowheadsupport.com>wrote: > All, > > I apologize in advance for what appears to be a question asked quite > often, but I am not sure I have ever seen an answer that explains it. This > may also be a bit long-winded so I apologize for that as well. > > I would like to know how much unique space each individual snapshot is > using. > > I have a ZFS filesystem that shows: > > $zfs list -o space rootpool/export/home > NAME AVAIL USED USEDSNAP USEDDS > USEDREFRESERV USEDCHILD > rootpool/export/home 5.81G 14.4G 8.81G 5.54G 0 > 0 > > So reading this I see that I have a total of 14.4G of space used by this > data set. Currently 5.54 is "active" data that is available on the normal > filesystem and 8.81G used in snapshots. 8.81G + 5.54G = 14.4G (roughly). > I 100% agree with these numbers and the world makes sense. > > This is also backed up by: > > $zfs get usedbysnapshots rootpool/export/home > NAME PROPERTY VALUE > SOURCE > rootpool/export/home usedbysnapshots 8.81G - > > > Now if I wanted to see how much space any individual snapshot is currently > using I would like to think that this would show me: > > $zfs list -ro space rootpool/export/home > > NAME AVAIL USED > USEDSNAP USEDDS USEDREFRESERV USEDCHILD > rootpool/export/home 5.81G 14.4G 8.81G > 5.54G 0 0 > rootpool/export/home@week3 - 202M - - > - - > rootpool/export/home@week2 - 104M - - > - - > rootpool/export/home@7daysago - 1.37M - - > - - > rootpool/export/home@6daysago - 1.20M - - > - - > rootpool/export/home@5daysago - 1020K - - > - - > rootpool/export/home@4daysago - 342K - - > - - > rootpool/export/home@3daysago - 1.28M - - > - - > rootpool/export/home@week1 - 0 - > - - - > rootpool/export/home@2daysago - 0 - - > - - > rootpool/export/home@yesterday - 360K - - > - - > rootpool/export/home@today - 1.26M - > - - - > > > So normal logic would tell me if USEDSNAP is 8.81G and is composed of 11 > snapshots, I would add up the size of each of those snapshots and that > would roughly equal 8.81G. So time to break out the calculator: > > 202M + 104M + 1.37M + 1.20M + 1020K + 342K + 1.28M +0 +0 + 360K + 1.26M > equals....... ~312M! > > That is nowhere near 8.81G. I would accept it even if it was within 15%, > but it's not even close. That definitely not metadata or ZFS overhead or > anything. > > I understand that snapshots are just the delta between the time when the > snapshot was taken and the current "active" filesystem and are truly just > references to a block on disk rather than a "copy". I also understand how > two (or more) snapshots can reference the same block on a disk but yet > there is still only that one block used. If I delete a recent snapshot I > may not save as much space as advertised because some may be inherited by a > "parent" snapshot. But that inheritance is not creating duplicate used > space on disk so it doesn't justify the huge difference in sizes. > > But even with this logic in place there is currently 8.81G of blocks > referred to by snapshots which are not currently on the "active" filesystem > and I don't believe anyone can argue with that. Can something show me how > much space a single snapshot has reserved? > > I searched through some of the archives and found this thread ( > http://mail.opensolaris.org/pipermail/zfs-discuss/2012-August/052163.html) > from early this month and I feel as if I have the same problem as the OP, > but hopefully attacking it with a little more background. I am not arguing > with discrepancies between df/du and zfs output and I have read the Oracle > documentation about it but haven't found what I feel like should be a > simple answer. I currently have a ticket open with Oracle, but I am > getting answers to all kinds of questions except for the question I am > asking so I am hoping someone out there might be able to help me. > > I am a little concerned I am going to find out that there is no real way > to show it and that makes for one sad SysAdmin. > > Thanks, > Chad > > > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss >
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