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.