hammer_blockmap_layer1
struct hammer_blockmap_layer1 layer1;
struct hammer_blockmap_layer1 layer1;
struct hammer_blockmap_layer1 layer1;
assert(sizeof(struct hammer_blockmap_layer1) == 32);
offsetof(struct hammer_blockmap_layer1, layer1_crc)
((int)(HAMMER_BIGBLOCK_SIZE / sizeof(struct hammer_blockmap_layer1)))
sizeof(struct hammer_blockmap_layer1))