da(4)

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-op

tical, 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.

PARTITIONING

The da driver allows the disk to have two levels of parti

tioning. One
layer, called the ``slice layer'', is used to separate the

FreeBSD areas
of the disk from areas used by other operating systems. The

second layer
is the native 4.4BSD partitioning scheme, disklabel(5),

which is used to
subdivide the FreeBSD slices into areas for individual file

systems and
swap spaces. For more information, see fdisk(8) and diskla

bel(8),
respectively.

If an uninitialized disk is opened, the slice table will be

initialized
with a fictitious FreeBSD slice spanning the entire disk.

Similarly, if
an uninitialized (or non-FreeBSD) slice is opened, its

disklabel will be
initialized with parameters returned by the drive and a sin

gle `c' partition encompassing the entire slice.

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 modi

fied 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 ef

fect. Most
devices with a read cache come from the factory with it en

abled. 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 efficien

cy 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 de

vices age
write transactions to limit vulnerability to a few transac

tions recently
reported as complete, but it is none-the-less recommended

that systems
with write cache enabled devices reside on an Uninterrupt

ible 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 re

duce the number and length of seeks. To ensure that transactions to

distant portions
of the media, which may be deferred indefinitely by servic

ing 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, respective

ly, 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.

IOCTLS

The following ioctl(2) calls apply to SCSI disks as well as

to other
disks. They are defined in the header file

DIOCSBAD Usually used to set up a bad-block mapping sys

tem on the

disk. SCSI drives incorporate their own bad

block mapping so
this command is not implemented.

DIOCGDINFO Read, from the kernel, the in-core copy of the

disklabel for

the drive. This may be a fictitious disklabel

if the drive
has never been initialized, in which case it

will contain
information read from the SCSI inquiry commands.

DIOCSDINFO Give the driver a new disklabel to use. The

driver will not

write the new disklabel to the disk.

DIOCWLABEL Enable or disable the driver's software write

protect of the

disklabel on the disk.

DIOCWDINFO Give the driver a new disklabel to use. The

driver will

write the new disklabel to the disk.

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 rou

tine. 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 val

ue 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 ker

nel 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/dausn raw mode SCSI disk unit u, slice n, accessed

as an unpar

titioned device

/dev/daup raw mode SCSI disk unit u, first FreeBSD

slice, partition

p

/dev/dausnp raw mode SCSI disk unit u, nth slice, parti

tion p

DIAGNOSTICS

None.

SEE ALSO

ad(4), disklabel(5), disklabel(8), fdisk(8)

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. Support for slices was written

by Bruce
Evans.

BSD September 2, 2003

BSD