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
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