Manual browser: disk(9)

DISK(9) Kernel Developer's Manual DISK(9)


disk, disk_init, disk_attach, disk_begindetach, disk_detach, disk_destroy, disk_busy, disk_unbusy, disk_isbusy, disk_find, disk_blocksizegeneric disk framework


#include <sys/types.h>
#include <sys/disklabel.h>
#include <sys/disk.h>

disk_init(struct disk *, const char *name, const struct dkdriver *driver);

disk_attach(struct disk *);

disk_begindetach(struct disk *, int (*lastclose)(device_t), device_t self, int flags);

disk_detach(struct disk *);

disk_destroy(struct disk *);

disk_busy(struct disk *);

disk_unbusy(struct disk *, long bcount, int read);

disk_isbusy(struct disk *);

struct disk *
disk_find(const char *);

disk_blocksize(struct disk *, int blocksize);


The NetBSD generic disk framework is designed to provide flexible, scalable, and consistent handling of disk state and metrics information. The fundamental component of this framework is the disk structure, which is defined as follows:

struct disk { 
	TAILQ_ENTRY(disk) dk_link;	/* link in global disklist */ 
	const char	*dk_name;	/* disk name */ 
	prop_dictionary_t dk_info;	/* reference to disk-info dictionary */ 
	int		dk_bopenmask;	/* block devices open */ 
	int		dk_copenmask;	/* character devices open */ 
	int		dk_openmask;	/* composite (bopen|copen) */ 
	int		dk_state;	/* label state   ### */ 
	int		dk_blkshift;	/* shift to convert DEV_BSIZE to blks */ 
	int		dk_byteshift;	/* shift to convert bytes to blks */ 
	 * Metrics data; note that some metrics may have no meaning 
	 * on certain types of disks. 
	struct io_stats	*dk_stats; 
	const struct dkdriver *dk_driver;	/* pointer to driver */ 
	 * Information required to be the parent of a disk wedge. 
	kmutex_t	dk_rawlock;	/* lock on these fields */ 
	u_int		dk_rawopens;	/* # of openes of rawvp */ 
	struct vnode	*dk_rawvp;	/* vnode for the RAW_PART bdev */ 
	kmutex_t	dk_openlock;	/* lock on these and openmask */ 
	u_int		dk_nwedges;	/* # of configured wedges */ 
					/* all wedges on this disk */ 
	LIST_HEAD(, dkwedge_softc) dk_wedges; 
	 * Disk label information.  Storage for the in-core disk label 
	 * must be dynamically allocated, otherwise the size of this 
	 * structure becomes machine-dependent. 
	daddr_t		dk_labelsector;		/* sector containing label */ 
	struct disklabel *dk_label;	/* label */ 
	struct cpu_disklabel *dk_cpulabel; 

The system maintains a global linked-list of all disks attached to the system. This list, called disklist, may grow or shrink over time as disks are dynamically added and removed from the system. Drivers which currently make use of the detachment capability of the framework are the ccd, dm, and vnd pseudo-device drivers.

The following is a brief description of each function in the framework:

Initialize the disk structure.
Attach a disk; allocate storage for the disklabel, set the “attached time” timestamp, insert the disk into the disklist, and increment the system disk count.
Check whether the disk is open, and if not, return 0. If the disk is open, and DETACH_FORCE is not set in flags, return EBUSY. Otherwise, call the provided lastclose routine (if not NULL) and return its exit code.
Detach a disk; free storage for the disklabel, remove the disk from the disklist, and decrement the system disk count. If the count drops below zero, panic.
Release resources used by the disk structure when it is no longer required.
Increment the disk's “busy counter”. If this counter goes from 0 to 1, set the timestamp corresponding to this transfer.
Decrement a disk's busy counter. If the count drops below zero, panic. Get the current time, subtract it from the disk's timestamp, and add the difference to the disk's running total. Set the disk's timestamp to the current time. If the provided byte count is greater than 0, add it to the disk's running total and increment the number of transfers performed by the disk. The third argument read specifies the direction of I/O; if non-zero it means reading from the disk, otherwise it means writing to the disk.
Returns true if disk is marked as busy and false if it is not.
Return a pointer to the disk structure corresponding to the name provided, or NULL if the disk does not exist.
Initialize dk_blkshift and dk_byteshift members of struct disk with suitable values derived from the supplied physical blocksize. It is only necessary to call this function if the device's physical blocksize is not DEV_BSIZE.

The functions typically called by device drivers are disk_init() disk_attach(), disk_begindetach(), disk_detach(), disk_destroy(), disk_busy(), disk_unbusy(), and disk_blocksize(). The function disk_find() is provided as a utility function.


The following ioctls should be implemented by disk drivers:
DIOCGDINFO struct disklabel
Get disklabel.
DIOCSDINFO struct disklabel
Set in-memory disklabel.
DIOCWDINFO struct disklabel
Set in-memory disklabel and write on-disk disklabel.
DIOCGPART struct partinfo
Get partition information. This is used internally.
DIOCRFORMAT struct format_op
Read format.
DIOCWFORMAT struct format_op
Write format.
Set step rate.
Set number of retries.
Specify whether to keep or drop the in-memory disklabel when the device is closed.
Enable or disable writing to the part of the disk that contains the label.
DIOCSBAD struct dkbad
Set kernel dkbad.
Eject removable disk.
Lock or unlock disk pack. For devices with removable media, locking is intended to prevent the operator from removing the media.
DIOCGDEFLABEL struct disklabel
Get default label.
Clear disk label.
Get status of disk read and write caches. The result is a bitmask containing the following values:
Read cache enabled.
Write(back) cache enabled.
Read cache enable is changeable.
Write cache enable is changeable.
Cache parameters may be saved, so that they persist across reboots or device detach/attach cycles.
Set status of disk read and write caches. The input is a bitmask in the same format as used for DIOCGCACHE.
Synchronise the disk cache. This causes information in the disk's write cache (if any) to be flushed to stable storage. The argument specifies whether or not to force a flush even if the kernel believes that there is no outstanding data.
DIOCBSLIST struct disk_badsecinfo
Get bad sector list.
Flush bad sector list.
DIOCAWEDGE struct dkwedge_info
Add wedge.
DIOCGWEDGEINFO struct dkwedge_info
Get wedge information.
DIOCDWEDGE struct dkwedge_info
Delete wedge.
DIOCLWEDGES struct dkwedge_list
List wedges.
DIOCGSTRATEGY struct disk_strategy
Get disk buffer queue strategy.
DIOCSSTRATEGY struct disk_strategy
Set disk buffer queue strategy.
DIOCGDISKINFO struct plistref
Get disk-info dictionary.
Get disk size in bytes.
Get sector size in bytes.


This section includes a description on basic use of the framework and example usage of its functions. Actual implementation of a device driver which uses the framework may vary.

Each device in the system uses a “softc” structure which contains autoconfiguration and state information for that device. In the case of disks, the softc should also contain one instance of the disk structure, e.g.:

struct foo_softc { 
	device_t	sc_dev;		/* generic device information */ 
	struct	disk	sc_dk;		/* generic disk information */ 
	[ . . . more . . . ] 

In order for the system to gather metrics data about a disk, the disk must be registered with the system. The disk_attach() routine performs all of the functions currently required to register a disk with the system including allocation of disklabel storage space, recording of the time since boot that the disk was attached, and insertion into the disklist. Note that since this function allocates storage space for the disklabel, it must be called before the disklabel is read from the media or used in any other way. Before disk_attach() is called, a portions of the disk structure must be initialized with data specific to that disk. For example, in the “foo” disk driver, the following would be performed in the autoconfiguration “attach” routine:

fooattach(device_t parent, device_t self, void *aux) 
	struct foo_softc *sc = device_private(self); 
	[ . . . ] 
	/* Initialize and attach the disk structure. */ 
	disk_init(&sc->sc_dk, device_xname(self), &foodkdriver); 
	/* Read geometry and fill in pertinent parts of disklabel. */ 
	[ . . . ] 
	disk_blocksize(&sc->sc_dk, bytes_per_sector); 

The foodkdriver above is the disk's “driver” switch. This switch currently includes a pointer to the disk's “strategy” routine. This switch needs to have global scope and should be initialized as follows:

void foostrategy(struct buf *); 
const struct dkdriver foodkdriver = { 
	.d_strategy = foostrategy, 

Once the disk is attached, metrics may be gathered on that disk. In order to gather metrics data, the driver must tell the framework when the disk starts and stops operations. This functionality is provided by the disk_busy() and disk_unbusy() routines. Because struct disk is part of device driver private data it needs to be guarded. Mutual exclusion must be done by driver disk_busy() and disk_unbusy() are not thread safe. The disk_busy() routine should be called immediately before a command to the disk is sent, e.g.:

	struct foo_softc *sc; 
	[ . . . ] 
	/* Get buffer from drive's transfer queue. */ 
	[ . . . ] 
	/* Build command to send to drive. */ 
	[ . . . ] 
	/* Tell the disk framework we're going busy. */ 
	/* Send command to the drive. */ 
	[ . . . ] 

When disk_busy() is called, a timestamp is taken if the disk's busy counter moves from 0 to 1, indicating the disk has gone from an idle to non-idle state. At the end of a transaction, the disk_unbusy() routine should be called. This routine performs some consistency checks, such as ensuring that the calls to disk_busy() and disk_unbusy() are balanced. This routine also performs the actual metrics calculation. A timestamp is taken and the difference from the timestamp taken in disk_busy() is added to the disk's total running time. The disk's timestamp is then updated in case there is more than one pending transfer on the disk. A byte count is also added to the disk's running total, and if greater than zero, the number of transfers the disk has performed is incremented. The third argument read specifies the direction of I/O; if non-zero it means reading from the disk, otherwise it means writing to the disk.

	struct foo_xfer *xfer; 
	struct foo_softc = (struct foo_softc *)xfer->xf_softc; 
	struct buf *bp = xfer->xf_buf; 
	long nbytes; 
	[ . . . ] 
	 * Get number of bytes transferred.  If there is no buf 
	 * associated with the xfer, we are being called at the 
	 * end of a non-I/O command. 
	if (bp == NULL) 
		nbytes = 0; 
		nbytes = bp->b_bcount - bp->b_resid; 
	[ . . . ] 
	/* Notify the disk framework that we've completed the transfer. */ 
	disk_unbusy(&sc->sc_dk, nbytes, 
	    bp != NULL ? bp->b_flags & B_READ : 0); 
	[ . . . ] 

disk_isbusy() is used to get status of disk device it returns true if device is currently busy and false if it is not. Like disk_busy() and disk_unbusy() it requires explicit locking from user side.


The disk framework itself is implemented within the file sys/kern/subr_disk.c. Data structures and function prototypes for the framework are located in sys/sys/disk.h.

The NetBSD machine-independent SCSI disk and CD-ROM drivers use the disk framework. They are located in sys/scsi/sd.c and sys/scsi/cd.c.

The NetBSD ccd, dm, and vnd drivers use the detachment capability of the framework. They are located in sys/dev/ccd.c, sys/dev/vnd.c, and sys/dev/dm/device-mapper.c.


ccd(4), dm(4), vnd(4)


The NetBSD generic disk framework appeared in NetBSD 1.2.


The NetBSD generic disk framework was architected and implemented by Jason R. Thorpe <>.
December 30, 2009 NetBSD 7.0