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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.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
         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

     ad(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.