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DA(4)		       FreeBSD Kernel Interfaces Manual 		 DA(4)

NAME
     da -- SCSI Direct Access device driver

SYNOPSIS
     device da
     device da1 at scbus0 target 4 unit 0

DESCRIPTION
     The da driver provides support for all SCSI devices of the direct access
     class that are attached to the system through a supported SCSI Host
     Adapter.  The direct access class includes disk, magneto-optical, and
     solid-state devices.

     A SCSI Host adapter must also be separately configured into the system
     before a SCSI direct access device can be configured.

PARTITIONING
     The da driver allows the disk to have two levels of partitioning.	One
     layer, called the ``slice layer'', is used to separate the FreeBSD areas
     of the disk from areas used by other operating systems.  The second layer
     is the native 4.4BSD partitioning scheme, disklabel(5), which is used to
     subdivide the FreeBSD slices into areas for individual filesystems and
     swap spaces.  For more information, see fdisk(8) and disklabel(8),
     respectively.

     If an uninitialized disk is opened, the slice table will be initialized
     with a fictitious FreeBSD slice spanning the entire disk.	Similarly, if
     an uninitialized (or non-FreeBSD) slice is opened, its disklabel will be
     initialized with parameters returned by the drive and a single `c' parti-
     tion encompassing the entire slice.

CACHE EFFECTS
     Many direct access devices are equipped with read and/or write caches.
     Parameters affecting the device's cache are stored in mode page 8, the
     caching control page.  Mode pages can be examined and modified via the
     camcontrol(8) utility.

     The read cache is used to store data from device-initiated read ahead
     operations as well as frequently used data.  The read cache is transpar-
     ent to the user and can be enabled without any adverse effect.  Most
     devices with a read cache come from the factory with it enabled.  The
     read cache can be disabled by setting the RCD (Read Cache Disable) bit in
     the caching control mode page.

     The write cache can greatly decrease the latency of write operations and
     allows the device to reorganize writes to increase efficiency and perfor-
     mance.  This performance gain comes at a price.  Should the device lose
     power while its cache contains uncommitted write operations, these writes
     will be lost.  The effect of a loss of write transactions on a file sys-
     tem is non-deterministic and can cause corruption.  Most devices age
     write transactions to limit vulnerability to a few transactions recently
     reported as complete, but it is none-the-less recommended that systems
     with write cache enabled devices reside on an Uninterruptible Power Sup-
     ply (UPS).  The da device driver ensures that the cache and media are
     synchronized upon final close of the device or an unexpected shutdown
     (panic) event.  This ensures that it is safe to disconnect power once the
     operating system has reported that it has halted.	The write cache can be
     enabled by setting the WCE (Write Cache Enable) bit in the caching con-
     trol mode page.

TAGGED QUEUING
     The da device driver will take full advantage of the SCSI feature known
     as tagged queueing.  Tagged queueing allows the device to process multi-
     ple transactions concurrently, often re-ordering them to reduce the num-
     ber and length of seeks.  To ensure that transactions to distant portions
     of the media, which may be deferred indefinitely by servicing requests
     nearer the current head position, are completed in a timely fashion, an
     ordered tagged transaction is sent every 15 seconds during continuous
     device operation.

BAD BLOCK RECOVERY
     Direct Access devices have the capability of mapping out portions of
     defective media.  Media recovery parameters are located in mode page 1,
     the Read-Write Error Recovery mode page.  The most important media remap-
     ping features are 'Auto Write Reallocation' and 'Auto Read Reallocation'
     which can be enabled via the AWRE and ARRE bits, respectively, of the
     Read-Write Error Recovery page.  Many devices do not ship from the fac-
     tory with these feature enabled.  Mode pages can be examined and modifie
     via the camcontrol(8) utility.

KERNEL CONFIGURATION
     It is only necessary to explicitly configure one da device; data struc-
     tures are dynamically allocated as disks are found on the SCSI bus.

IOCTLS
     The following ioctl(2) calls apply to SCSI disks as well as to other
     disks.  They are defined in the header file <sys/disklabel.h>.

     DIOCSBAD	 Usually used to set up a bad-block mapping system on the
		 disk.	SCSI drives incorporate their own bad-block mapping so
		 this command is not implemented.

     DIOCGDINFO  Read, from the kernel, the in-core copy of the disklabel for
		 the drive.  This may be a fictitious disklabel if the drive
		 has never been initialized, in which case it will contain
		 information read from the SCSI inquiry commands.

     DIOCSDINFO  Give the driver a new disklabel to use.  The driver will not
		 write the new disklabel to the disk.

     DIOCWLABEL  Enable or disable the driver's software write protect of the
		 disklabel on the disk.

     DIOCWDINFO  Give the driver a new disklabel to use.  The driver will
		 write the new disklabel to the disk.

SYSCTL VARIABLES
     The following variables are available as both sysctl(8) variables and
     loader(8) tunables:

     kern.cam.da.retry_count

	 This variable determines how many times the da driver will retry a
	 READ or WRITE command.  This does not affect the number of retries
	 used during probe time or for the da driver dump routine.  This value
	 currently defaults to 4.

     kern.cam.da.default_timeout

	 This variable determines how long the da driver will wait before tim-
	 ing out an outstanding command.  The units for this value are sec-
	 onds, and the default is currently 60 seconds.

     kern.cam.da.%d.minimum_cmd_size

	 This variable determines what the minimum READ/WRITE CDB size is for
	 a given da unit.  (The %d above denotes the unit number of the da
	 driver instance, e.g. 1, 2, 4, 8, etc.)  Valid minimum command size
	 values are 6, 10, 12 and 16 bytes.  The default is 6 bytes.

	 The da driver issues a CAM Path Inquiry CCB at probe time to deter-
	 mine whether the protocol the device in question speaks (e.g. ATAPI)
	 typically doesn't allow 6 byte commands.  If it doesn't, the da
	 driver will default to using at least 10 byte CDBs.  If a 6 byte READ
	 or WRITE fails with an ILLEGAL REQUEST error, the da driver will then
	 increase the default CDB size for the device to 10 bytes and retry
	 the command.  CDB size is always chosen as the smallest READ/WRITE
	 CDB that will satisfy the specified minimum command size, and the LBA
	 and length of the READ or WRITE in question.  (e.g., a write to an
	 LBA larger than 2^32 will require a 16 byte CDB.)

NOTES
     If a device becomes invalidated (media is removed, device becomes unre-
     sponsive) the disklabel and information held within the kernel about the
     device will be invalidated.  To avoid corruption of a newly inserted
     piece of media or a replacement device, all accesses to the device will
     be discarded until the last file descriptor referencing the old device is
     closed.  During this period, all new open attempts will be rejected.

FILES
     /dev/dausn     raw mode SCSI disk unit u, slice n, accessed as an unpar-
		    titioned device
     /dev/daup	    raw mode SCSI disk unit u, first FreeBSD slice, partition
		    p
     /dev/dausnp    raw mode SCSI disk unit u, nth slice, partition p

DIAGNOSTICS
     None.

SEE ALSO
     ad(4), disklabel(5), disklabel(8), fdisk(8)

HISTORY
     The da driver was written for the CAM SCSI subsystem by Justin T. Gibbs.
     Many ideas were gleaned from the sd device driver written and ported from
     Mach 2.5 by Julian Elischer.  Support for slices was written by Bruce
     Evans.

FreeBSD 4.9		       September 2, 2003		   FreeBSD 4.9