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

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.\"	Julian Elischer <julian@FreeBSD.org>.  All rights reserved.
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.Dd October 15, 1998
.Dt DA 4
.Os FreeBSD
.Sh NAME
.Nm da
.Nd SCSI Direct Access Device Driver
.Sh SYNOPSIS
.Cd device da
.Cd device da1 at scbus0 target 4 unit 0
.Sh DESCRIPTION
The
.Nm da
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 PARTITIONING
The
.Nm
driver allows the disk to have two levels of partitioning.
One layer, called the
.Dq slice layer ,
is used to separate the
.Tn FreeBSD
areas of the disk from areas used by other operating systems.
The second layer is the native
.Bx 4.4
partitioning scheme,
.Xr disklabel 5 ,
which is used to subdivide the
.Tn FreeBSD
slices into areas for individual filesystems and swap spaces.
For more information, see
.Xr fdisk 8
and
.Xr disklabel 8 ,
respectively.
.Pp
If an uninitialized disk is opened, the slice table will be
initialized with a fictitious
.Tn FreeBSD
slice spanning the entire disk.  Similarly, if an uninitialized
(or
.No non- Ns Tn FreeBSD )
slice is opened, its disklabel will be initialized with parameters returned
by the drive and a single
.Sq Li c
partition encompassing the entire slice.
.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.

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.

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 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
.Tn WCE
(Write Cache Enable) bit in the caching control mode page.
.Sh TAGGED QUEUING
The
.Nm da
device driver will take full advantage of the SCSI feature known as tagged
queuing.  Tagged queuing 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 modifie
via the
.Xr camcontrol 8
utility.
.Sh KERNEL CONFIGURATION
It is only necessary to explicitly configure one
.Nm da
device; data structures are dynamically allocated as disks are found
on the
.Tn SCSI
bus.
.Sh IOCTLS
The following
.Xr ioctl 2
calls apply to
.Tn SCSI
disks as well as to other disks.  They are defined in the header file
.Aq Pa sys/disklabel.h .
.Pp
.Bl -tag -width DIOCSDINFO
.It Dv DIOCSBAD
Usually used to set up a bad-block mapping system on the disk.
.Tn SCSI
drives incorporate their own bad-block mapping so this command is not
implemented.
.It Dv 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
.Tn SCSI
inquiry commands.
.It Dv DIOCSDINFO
Give the driver a new disklabel to use.
The driver
.Em will not
write the new
disklabel to the disk.
.It Dv DIOCWLABEL
Enable or disable the driver's software
write protect of the disklabel on the disk.
.It Dv DIOCWDINFO
Give the driver a new disklabel to use.
The driver
.Em will
write the new disklabel to the disk.
.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 /dev/rsdXXXXX -compact
.It Pa /dev/rda Ns Ar u
raw mode
.Tn SCSI
disk unit
.Ar u ,
accessed as an unpartitioned device
.Sm off
.It Pa /dev/da Ar u Pa s Ar n
.Sm on
block mode
.Tn SCSI
disk unit
.Ar u ,
slice
.Ar n ,
accessed as an unpartitioned device
.Sm off
.It Pa /dev/rda Ar u Pa s Ar n
.Sm on
raw mode
.Tn SCSI
disk unit
.Ar u ,
slice
.Ar n ,
accessed as an unpartitioned device
.It Pa /dev/da Ns Ar u Ns Ar p
block mode
.Tn SCSI
disk unit
.Ar u ,
first
.Tn FreeBSD
slice, partition
.Ar p
.It Pa /dev/rda Ns Ar u Ns Ar p
raw mode
.Tn SCSI
disk unit
.Ar u ,
first
.Tn FreeBSD
slice, partition
.Ar p
.Sm off
.It Xo
.Pa /dev/da
.Ar u
.Pa s
.Ar n
.Ar p
.Xc
.Sm on
block mode
.Tn SCSI
disk unit
.Ar u ,
.No Ar n Ns th
slice, partition
.Ar p
.Sm off
.It Xo
.Pa /dev/rda
.Ar u
.Pa s
.Ar n
.Ar p
.Xc
.Sm on
raw mode
.Tn SCSI
disk unit
.Ar u ,
.No Ar n Ns th
slice, partition
.Ar p
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr ad 4 ,
.Xr disklabel 5 ,
.Xr disklabel 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 .
Support for slices was written by
.An Bruce Evans .