rsync problem

[Guy Van Sanden]

Hello

I'm using rsync to sort of mirror two 40GB disks (once a day).
All partitions work as expected, but root is weird (and as you can see
below, I sort of made it too small).

I use this command:
/usr/local/bin/rsync -ax --delete / /mirror/rootfs

But this is what I'm getting:
df -m
Filesystem  1M-blocks Used Avail Capacity  Mounted on
/dev/ad0s1a       154   71    70    51%    /
/dev/ad1s1a       154  138     4    97%    /mirror/rootfs

So, there's a 67 MB difference between both.
I started out wite a cleanly formatted mirror (UFS2)

My system is FreeBSD 5.0 RELEASE-p11

Thanks for any help

Guy