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

NAME
     da	-- SCSI	Direct Access device driver

SYNOPSIS
     device da

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.

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 modified
     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.

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 does	not allow 6 byte commands.  If it does not, 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/da*  SCSI disk device	nodes

DIAGNOSTICS
     None.

SEE ALSO
     ada(4), cam(4), geom(4), bsdlabel(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.

FreeBSD	10.1		       February	8, 2012			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | CACHE EFFECTS | TAGGED QUEUING | BAD BLOCK RECOVERY | KERNEL CONFIGURATION | SYSCTL VARIABLES | NOTES | FILES | DIAGNOSTICS | SEE ALSO | HISTORY

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