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:
- 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.)
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
None.
ada(4),
cam(4),
geom(4),
nda(4),
gpart(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.