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DA(4)			     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  be-
     fore 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  sub-
     divide  the  FreeBSD slices into areas for individual file systems 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 ini-
     tialized with parameters returned by the drive and a single  `c'  partition
     encompassing the entire slice.

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

     The read cache is used to store data from device-initiated read ahead oper-
     ations as well as frequently used data.  The read cache is  transparent  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 system
     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 Supply (UPS).  The
     da device driver ensures that the cache and media are synchronized upon fi-
     nal close of the device or an unexpected shutdown (panic) event.  This  en-
     sures that it is safe to disconnect power once the operating system has re-
     ported  that  it has halted.  The write cache can be enabled by setting the
     WCE (Write Cache Enable) bit in the caching control 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 multiple
     transactions concurrently, often re-ordering them to reduce the number  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 defec-
     tive  media.   Media  recovery  parameters  are located in mode page 1, the
     Read-Write Error Recovery mode page.  The most  important	media  remapping
     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 factory with these
     feature enabled.  Mode pages can be examined and modified via  the  camcon-
     trol(8) utility.

KERNEL CONFIGURATION
     It is only necessary to explicitly configure one da device; data structures
     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	informa-
		 tion 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.

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

FILES
     /dev/rdau	    raw  mode SCSI disk unit u, accessed as an unpartitioned de-
		    vice
     /dev/dausn     block mode SCSI disk unit u, slice n, accessed as an  unpar-
		    titioned device
     /dev/rdausn    raw  mode SCSI disk unit u, slice n, accessed as an unparti-
		    tioned device
     /dev/daup	    block mode SCSI disk unit u, first FreeBSD slice,  partition
		    p
     /dev/rdaup     raw mode SCSI disk unit u, first FreeBSD slice, partition p
     /dev/dausnp    block mode SCSI disk unit u, nth slice, partition p
     /dev/rdausnp   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 5.0			October 15, 1998			   DA(4)
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