dump/mksnap_ffs & fsck failure

Hello all,

Is this likely to be filesystem corruption in a really bad way, or a
ufs bug:

# /sbin/dump -0 -LuaC32 -f /backup/root.dump /
mksnap_ffs: Cannot create //.snap/dump_snapshot: Input/output error

# fsck_ufs /dev/mirror/gma
** /dev/mirror/gma (NO WRITE)
** Last Mounted on /
** Root file system
** Phase 1 - Check Blocks and Sizes
fsck_ufs: cannot alloc 4294967292 bytes for inoinfo

A machine with similar aged sources (this is 5.3B6) is fine, however.
The only differences I've made to sources are to apply
root_mount.patch from pjd.

Making snapshots and fsck'ing other filesystems (also in a gmirror)
works fine..

-- 
Chris