3. Metadata update reaches disk but data does not. The interesting
case! The L2 table now points to a cluster which is beyond the last
cluster in the image file. Remember that file size is rounded down by
cluster size, so partial data writes are discarded and this case
applies.
Now we're in trouble. The image cannot be accessed without some
sanity checking because not only do table entries point to invalid
clusters, but new allocating writes might make previously invalid
cluster offsets valid again (then there would be two or more table
entries pointing to the same cluster)!
Anthony's suggestion is to use a "mounted" or "dirty" bit in the qed
header to detect a crashed image when opening the image file. If no
crash has occurred, then the mounted bit is unset and normal operation
is safe. If the mounted bit is set, then an check of the L1/L2 tables
must be performed and any invalid cluster offsets must be cleared to
zero. When an invalid cluster is cleared to zero, we arrive back at
case 1 above: neither data write nor metadata update reached the disk,
and we are in a safe state.