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da — SCSI Direct Access device driver
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
A SCSI Host adapter must also be separately configured into the system
before a SCSI direct access device can be configured.
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
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.
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.
It is only necessary to explicitly configure one da device; data
structures are dynamically allocated as disks are found on the SCSI bus.
The following variables are available as both sysctl(8) variables and
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.
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.
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.)
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.
/dev/da* SCSI disk device nodes
ad(4), cam(4), geom(4), bsdlabel(8), fdisk(8)
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.