int writeable;
int in_fs_metadata;
int missing;
+ int can_discard;
spinlock_t io_lock;
u64 rw_devices;
u64 missing_devices;
u64 total_rw_bytes;
+ u64 num_can_discard;
struct block_device *latest_bdev;
/* all of the devices in the FS, protected by a mutex