can ignore this option unless doing huge amounts of really
fast IO where the red-black tree sorting CPU time becomes
significant.
+
+header_interval=siint Write the verification header at a finer granularity
+ than the blocksize. It will be written for chunks the
+ size of header_interval. blocksize should divide this
+ evenly.
stonewall Wait for preceeding jobs in the job file to exit, before
starting this one. Can be used to insert serialization