1. We should not caclucate the superblock summary, since it can at most lag 30 seconds’ change, and doing the caclucate is painful delay. What we should do is to transfer the responsibility to background fsck
2. newfs(8) should warn about the snapshot is actually unable to create, in case fssize is greater than maxfilesize.

The patch for 1) is available at http://people.freebsd.org/~delphij/ffs/patch-sbsummary. I will work on 2) shortly, as it’s quite simpler than 1).