freebsd-nq/share/man/man4/da.4

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.\"	Julian Elischer <julian@FreeBSD.org>.  All rights reserved.
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.Dd February 8, 2012
.Dt DA 4
.Os
.Sh NAME
.Nm da
.Nd SCSI Direct Access device driver
.Sh SYNOPSIS
.Cd device da
.Sh DESCRIPTION
The
.Nm
driver provides support for all
.Tn SCSI
devices of the direct access class that are attached to the system
through a supported
.Tn SCSI
Host Adapter.
The direct access class includes disk, magneto-optical,
and solid-state devices.
.Pp
A
.Tn SCSI
Host
adapter must also be separately configured into the system
before a
.Tn SCSI
direct access device can be configured.
.Sh 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
.Xr camcontrol 8
utility.
.Pp
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
.Tn RCD
(Read Cache Disable) bit in the caching control mode page.
.Pp
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
.Nm
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
.Tn WCE
(Write Cache Enable) bit in the caching control mode page.
.Sh TAGGED QUEUING
The
.Nm
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.
.Sh 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
.Xr camcontrol 8
utility.
.Sh KERNEL CONFIGURATION
It is only necessary to explicitly configure one
.Nm
device; data structures are dynamically allocated as disks are found
on the
.Tn SCSI
bus.
.Sh SYSCTL VARIABLES
The following variables are available as both
.Xr sysctl 8
variables and
.Xr loader 8
tunables:
.Bl -tag -width 12
.It kern.cam.da.retry_count
.Pp
This variable determines how many times the
.Nm
driver will retry a READ or WRITE command.
This does not affect the number of retries used during probe time or for
the
.Nm
driver dump routine.
This value currently defaults to 4.
.It kern.cam.da.default_timeout
.Pp
This variable determines how long the
.Nm
driver will wait before timing out an outstanding command.
The units for this value are seconds, and the default is currently 60
seconds.
.It kern.cam.da.%d.minimum_cmd_size
.Pp
This variable determines what the minimum READ/WRITE CDB size is for a
given
.Nm
unit.
(The %d above denotes the unit number of the
.Nm
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.
.Pp
The
.Nm
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
.Nm
driver will default to using at least 10 byte CDBs.
If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
.Nm
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.)
.El
.Sh 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.
.Sh FILES
.Bl -tag -width ".Pa /dev/da*" -compact
.It Pa /dev/da*
SCSI disk device nodes
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr ada 4 ,
.Xr cam 4 ,
.Xr geom 4 ,
.Xr bsdlabel 8 ,
.Xr fdisk 8
.Sh HISTORY
The
.Nm
driver was written for the
.Tn CAM
.Tn SCSI
subsystem by
.An Justin T. Gibbs .
Many ideas were gleaned from the
.Nm sd
device driver written and ported from
.Tn Mach
2.5
by
.An Julian Elischer .