While the Berkeley FFS and derivatives like Solaris UFS are much, much better at dealing with fragmentation than the original V7 filesystem (which continued on into System V before SysVR4), they can actually get fragmented over time to a degree that matters.

Traditional Unix, Solaris included, really doesn't have any tools for defragmenting filesystems; instead, you get to to do it the brute force way, by copying everything into a clear filesystem. With Solaris Disksuite and mirrored disks, it is possible to do this 'in place', so that you don't have to copy twice or remount the filesystem on any NFS clients.

The procedure is slightly less nerve-wracking if you have a three way mirror, but should work OK even for a two-way mirror. Here's how it goes:

1. bring the machine into single-user mode, but do not unmount the filesystem you want to defragment.
2. insure that all the submirrors are in sync.
3. metadetach all but one submirror.
4. rm everything in the filesystem except the lost+found directory.
5. ufsdump 0f - /dev/md/rdsk/<detached submirror> | ufsrestore rf -
6. check and make sure the data is there. No, really, do it twice.
7. metattach the detached submirrors.
8. bring the machine back up multi-user. (Optionally, first wait for the resync to finish.)

You're done. NFS mounts are even still intact, although anyone with an open file or a current directory in a subdirectory of the filesystem will see a few small problems.

(In our case it was a mail spool, so there weren't any subdirectories to worry about.)

There should be a similar procedure with Linux software RAID, although it'll probably be slightly more troublesome since the Linux equivalent of metadetach is somewhat more abrupt. (I'd probably unmount the filesystem before ripping the mirror off.)

PS: I am not sure if I am grumpy that Sun didn't make 'metattach' be called 'metaattach' instead, for complete consistency among the Disksuite command names.