| DA(4) | Device Drivers Manual | DA(4) |
da — SCSI Direct
Access device driver
device da
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.
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.
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.
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 loader(8) tunables:
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.da driver
will wait before timing out an outstanding command. The units for this
value are seconds, and the default is currently 60 seconds.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.)
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.
None.
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.
| December 20, 2017 | Debian |