UFS snapshot "file" is slightly bigger than underlying disk partition

[Kirk McKusick]

> From: Maxim Sobolev <sobomax at freebsd.org>
> Date: Thu, 20 Apr 2017 02:39:12 -0700
> Subject: UFS snapshot "file" is slightly bigger than underlying disk partition
> To: FreeBSD Filesystems <freebsd-fs at freebsd.org>,
>         Kirk McKusick <mckusick at mckusick.com>
> 
> Hi Kirk,
> 
> I've noticed that the snapshot file is slightly bigger than underlying disk
> partition. First I thought it's some kind of header attached to the end,
> but the size of difference is actually dependent on the disk size. Is it by
> design, or some sort of "off by x" error? What's annoying about that is
> that the size is not multiple of SECTOR_SIZE. Also looks like if I just cut
> that junk out resulting FS image is just as usable.
> 
> Attached script illustrates that. The first column is size of the
> partition, the second column is the size of the difference, both in bytes.
> 
> 1048576 48
> 2097152 56
> 4194304 72
> 8388608 72
> 16777216 72
> 33554432 72
> 67108864 72
> 134217728 72
> 268435456 72
> 536870912 72
> 1073741824 72
> 2147483648 72
> 4294967296 96
> 8589934592 152
> 17179869184 256
> 34359738368 464
> 68719476736 880
> 137438953472 1720
> 274877906944 3392
> 549755813888 6744
> 
> Please advise, thanks.
> 
> -Max
> (P.S. This is 11.0-RELEASE-p9)

The extra space is for auxilary information that is used to track
changes made in the snapshot. Removing it will cause your snapshot
maintainance to slow down by 10-100x and in the worst case can
cause it to become corrupt. In short, don't mess with it.

	Kirk McKusick