My tests show that ffs1, at least, is broken with >2T filesystems.  I
know ffs1 has on-disk 32-bit values in places like the dblock and
iblock arrays, but I specifically built the filesystem 4K frags and 32K
blocks, and 2^32 frags of 4K each is 16T, while the filesystem is well
under 3T, so those on-disk values are within even *signed* 32-bit.

But when I wrote about 2.5T of nonrepeating but easy-to-recompute data,
I got corrupt data back on read.  Then, like an idiot, I tried to rm
-rf stuff, and the machine promptly panicked, presumably because some
inode block got stomped on with data.

I'm about to try ffsv2.

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