extent_ad
struct extent_ad main_vds_ex; /* to main volume descriptor set ; 16 sectors min */
struct extent_ad reserve_vds_ex; /* copy of main volume descriptor set ; 16 sectors min */
struct extent_ad next_vds_ex; /* points to the next block for volume descriptor space */
struct extent_ad vol_abstract;
struct extent_ad vol_copyright;
struct extent_ad integrity_seq_loc;
struct extent_ad alloc_desc[1];
struct extent_ad next_extent;