DragonFlyBSD

Ok I've tracked it down. There are two issues, both related to the
fact that the filesystem is really small.

The main bug is actually with newfs_hammer.  When a small filesystem
    is created newfs_hammer reserves way too small a space for the UNDO
    FIFO.  A 6G filesystem only reserves 8M of undo space!  That isn't
    enough.  100M is really the minimum.  I am going to change newfs_hammer
    to enforce that.

The second issue is that the filesystem is really too small.  I don't
    want to disallow the creation of small filesystem but I think I will
    also change newfs_hammer to spit out a bunch of warnings when people
    create such small filesystems.

Finally, keep in mind that you may hit up against serious free space
    issues and may have to run 'hammer prune-everything ...' as well
    as 'hammer reblock ...' quite often on such a small filesystem,
    depending on use.

-Matt
                    Matthew Dillon 
                    <dillon@backplane.com>