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 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 performance.  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 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 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 defective 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
       camcontrol(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.

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 timing out an  outstanding  command.
           The units for this value are seconds, and the default is currently 60 seconds.

       kern.cam.sort_io_queue

       kern.cam.da.X.sort_io_queue
           These  variables determine whether request queue should be sorted trying to optimize head seeks.  Set
           to 1 to enable sorting, 0 to disable, -1 to leave it as-is.  The default is sorting enabled for  HDDs
           and disabled for SSDs.

       kern.cam.da.X.delete_method
           This variable specifies method to handle BIO_DELETE requests:

           ATA_TRIM  ATA TRIM via ATA COMMAND PASS THROUGH command,

           UNMAP     UNMAP command,

           WS16      WRITE SAME(16) command with UNMAP flag,

           WS10      WRITE SAME(10) command with UNMAP flag,

           ZERO      WRITE SAME(10) command without UNMAP flag,

           DISABLE   disable BIO_DELETE support.

       kern.cam.da.X.minimum_cmd_size
           This  variable determines what the minimum READ/WRITE CDB size is for a given da unit.  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  determine  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 unresponsive) 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), nda(4), gpart(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.

Debian                                          December 20, 2017                                          DA(4)