DISK(9)

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

# NAME

`disk`, `disk_init`, `disk_attach`, `disk_begindetach`, `disk_detach`, `disk_destroy`, `disk_wait`, `disk_busy`, `disk_unbusy`, `disk_isbusy`, `disk_find`, `disk_set_info` —
generic disk framework

# SYNOPSIS

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

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

void
`disk_attach`(struct disk *);

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

void
`disk_detach`(struct disk *);

void
`disk_destroy`(struct disk *);

void
`disk_wait`(struct disk *);

void
`disk_busy`(struct disk *);

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

bool
`disk_isbusy`(struct disk *);

struct disk *
`disk_find`(const char *);

void
`disk_set_info`(device_t, struct disk *, const char *type);

# DESCRIPTION

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 opens 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:

`disk_init`()
  Initialize the disk structure.
`disk_attach`()
  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.
`disk_begindetach`()
  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.
`disk_detach`()
  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.
`disk_destroy`()
  Release resources used by the disk structure when it is no longer required.
`disk_wait`()
  Disk timings are measured by counting the number of queued requests (wait counter) and requests issued to the hardware (busy counter) and keeping timestamp when the counters change. The time interval between two changes of a counter is accumulated into a total and also multiplied by the counter value and the accumulated into a sum. Both values can be used to determine how much time is spent in the driver queue or in-flight to the hardware as well as the average number of requests in either state. `disk_wait`() increment the disk's wait counter and handles the accumulation.
`disk_busy`()
  Decrements the disk's wait counter and increments the disk's “busy counter”, and handles either accumulation. If the wait counter is still zero, it is assumed that the driver hasn't been updated to call `disk_wait`(), then only the values from the busy counter are available.
`disk_unbusy`()
  Decrement the disk's busy counter and handles the accumulation. 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.
`disk_isbusy`()
  Returns true if disk is marked as busy and false if it is not.
`disk_find`()
  Return a pointer to the disk structure corresponding to the name provided, or `NULL` if the disk does not exist.
`disk_set_info`()
  Setup disk-info dictionary and other dependent values of the disk structure, the driver must have initialized the dk_geom member of struct disk with suitable values. If type is not `NULL`, it will be added to the dictionary.

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

# DISK IOCTLS

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.
`DIOCSSTEP int`
  Set step rate.
`DIOCSRETRIES int`
  Set number of retries.
`DIOCKLABEL int`
  Specify whether to keep or drop the in-memory disklabel when the device is closed.
`DIOCWLABEL int`
  Enable or disable writing to the part of the disk that contains the label.
`DIOCSBAD struct dkbad`
  Set kernel dkbad.
`DIOCEJECT int`
  Eject removable disk.
`DIOCLOCK int`
  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.
`DIOCCLRLABEL`
  Clear disk label.
`DIOCGCACHE int`
  Get status of disk read and write caches. The result is a bitmask containing the following values:
  `DKCACHE_READ`
    Read cache enabled.
  `DKCACHE_WRITE`
    Write(back) cache enabled.
  `DKCACHE_RCHANGE`
    Read cache enable is changeable.
  `DKCACHE_WCHANGE`
    Write cache enable is changeable.
  `DKCACHE_SAVE`
    Cache parameters may be saved, so that they persist across reboots or device detach/attach cycles.
`DIOCSCACHE int`
  Set status of disk read and write caches. The input is a bitmask in the same format as used for `DIOCGCACHE`.
`DIOCCACHESYNC int`
  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.
`DIOCBSFLUSH`
  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.
`DIOCGMEDIASIZE off_t`
  Get disk size in bytes.
`DIOCGSECTORSIZE u_int`
  Get sector size in bytes.

# USING THE FRAMEWORK

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:

    void
    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);
    	disk_attach(&sc->sc_dk);

    	/* Read geometry and fill in pertinent parts of disklabel. */
    	/* Initialize geometry values of the disk structure */
    	[ . . . ]
    	disk_set_info(&self>, &sc->sc_dk, type);
    }

The `foodkdriver` above is the disk's “driver” switch. This switch currently includes pointers to several driver entry points, where only the `d_strategy` entry point is used by the disk framework. This switch needs to have global scope and should be initialized as follows:

    void    (foostrategy)(struct buf *);
    void    (foominphys)(struct buf *);
    int     (fooopen)(dev_t, int, int, struct lwp *);
    int     (fooclose)(dev_t, int, int, struct lwp *);
    int     (foo_discard)(device_t, off_t, off_t);
    int     (foo_diskstart)(device_t, struct buf *);
    void    (foo_iosize)(device_t, int *);
    int     (foo_dumpblocks)(device_t, void *, daddr_t, int);
    int     (foo_lastclose)(device_t);
    int     (foo_firstopen)(device_t, dev_t, int, int);
    int     (foo_label)(device_t, struct disklabel *);

    const struct dkdriver foodkdriver = {
    	.d_open = fooopen,
    	.d_close = fooclose,
    	.d_strategy = foostrategy,
    	.d_minphys = foominphys,
    	.d_discard = foo_discard,
    	.d_diskstart = foo_diskstart,	/* optional */
    	.d_dumpblocks = foo_dumpblocks,	/* optional */
    	.d_iosize = foo_iosize,		/* optional */
    	.d_firstopen = foo_firstopen,	/* optional */
    	.d_lastclose = foo_lastclose,	/* optional */
    	.d_label = foo_label,		/* optional */
    };

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 queues, starts and stops operations. This functionality is provided by the `disk_wait`(), `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_wait`(), `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.:

    void
    foostrategy(struct buf *bp)
    {
    	[ . . . ]

    	mutex_enter(&sc->sc_dk_mtx);
    	disk_wait(&sc->sc_dk);

    	/* Put buffer onto drive's transfer queue */

    	mutex_exit(&sc->sc_dk_mtx);

    	foostart(sc);
    }

    void
    foostart(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. */
    	mutex_enter(&sc->sc_dk_mtx);
    	disk_busy(&sc->sc_dk);
    	mutex_exit(&sc->sc_dk_mtx);

    	/* Send command to the drive. */
    	[ . . . ]
    }

The routine `disk_unbusy`() performs some consistency checks, such as ensuring that the calls to `disk_busy`() and `disk_unbusy`() are balanced. It also performs the final steps of the metrics calcuation. A byte count is 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.

    void
    foodone(xfer)
    	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;
    	else
    		nbytes = bp->b_bcount - bp->b_resid;

    	[ . . . ]

    	mutex_enter(&sc->sc_dk_mtx);
    	/* Notify the disk framework that we've completed the transfer. */
    	disk_unbusy(&sc->sc_dk, nbytes,
    	    bp != NULL ? bp->b_flags & B_READ : 0);
    	mutex_exit(&sc->sc_dk_mtx);

    	[ . . . ]
    }

`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_wait`(), `disk_busy`() and `disk_unbusy`() it requires explicit locking from user side.

# CODE REFERENCES

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.

# SEE ALSO

ccd(4), dm(4), vnd(4), dksubr(9)

# HISTORY

The NetBSD generic disk framework appeared in NetBSD 1.2.

# AUTHORS

The NetBSD generic disk framework was architected and implemented by Jason R. Thorpe <thorpej@NetBSD.org>.

March 5, 2017 | NetBSD 9.5