is there any other/better way to defragment the ufs slice instead of backup+restore?
how to tell which slice needs defragmenting?
Usually ufs does not need a lot of defragmentation done to it. If disks are approaching full, i.e. > 90% you can start having problems. If you are on a SAN there is virtually never a worry about fragmentation.
The only real issue with "fragmentation" relates to having a single directory with huge numbers of small files and loads of deletions/additions. This sometimes kills performance on the directory and also the filesystem itself.
a-ha, we have such directory. I'll tar it and remove files. they are some sort of logs, very short text files. thanks.