Ubuntu Manpage: da — SCSI Direct Access device driver

Provided by: freebsd-manpages_9.2+1-1_all

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.

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                                          February 8, 2012                                           DA(4)