Update the da(4) man page for CAM. The da.4 man page was repository-copied
from the sd.4 man page. This includes a discussion of write caching and its effects. Reviewed by: ken Submitted by: gibbs
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ken
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@ -23,71 +23,36 @@
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $Id: sd.4,v 1.10 1997/02/22 13:24:42 peter Exp $
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.\" $Id: sd.4,v 1.11 1997/04/01 04:04:09 jdp Exp $
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.\"
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.Dd January 18, 1996
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.Dt SD 4
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.Dd October 15, 1998
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.Dt DA 4
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.Os FreeBSD
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.Sh NAME
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.Nm sd
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.Nd SCSI disk driver
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.Nm da
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.Nd SCSI Direct Access Device Driver
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.Sh SYNOPSIS
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.Cd disk sd
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.Cd disk sd1 at scbus0 target 4 lun 0
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.Cd device da0
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.Cd device da1 at scbus0 target 4 lun 0
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.Sh DESCRIPTION
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The
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.Nm sd
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driver provides support for a
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.Nm da
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driver provides support for all
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.Tn SCSI
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disk. It allows the disk
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to be divided up into a set of pseudo devices called
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.Em partitions .
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In general the interfaces are similar to those described by
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.Xr wd 4 .
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devices of the direct access class that are attached to the system
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through a supported
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.Tn SCSI
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Host Adapter.
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The direct access class includes disk, magneto-optical,
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and solid-state devices.
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.Pp
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Where the
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.Xr wd 4
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device has a fairly low level interface to the system,
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.Tn SCSI
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devices have a much higher level interface and talk to the system via
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a
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.Tn SCSI
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host adapter
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(e.g.,
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.Xr ahc 4 ) .
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A
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.Tn SCSI
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Host
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adapter must also be separately configured into the system
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before a
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.Tn SCSI
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disk can be configured.
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.Pp
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When the
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.Tn SCSI
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adapter is probed during boot, the
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.Tn SCSI
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bus is scanned for devices. Any devices found which answer as
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.Sq Em Direct
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type devices will be attached to the
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.Nm
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driver.
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In
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.Tn FreeBSD
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releases prior to 2.1, the first found was attached as
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.Li sd0 ,
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the second
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.Li sd1 ,
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and so on.
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Beginning in
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.Fx 2.1
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it became possible to lock down the assignment of
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devices on the
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.Tn SCSI
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bus to particular units of the
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.Nm
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device; refer to
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.Xr scsi 4
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for details on kernel configuration.
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direct access device can be configured.
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.Sh PARTITIONING
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The
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.Nm
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@ -108,10 +73,10 @@ For more information, see
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.Xr fdisk 8
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and
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.Xr disklabel 8 ,
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respectively.)
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respectively.
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.Pp
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If an uninitialized disk is opened, the slice table will be
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initialized with a fictitious
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initialized with a ficticious
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.Tn FreeBSD
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slice spanning the entire disk. Similarly, if an uninitialized
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(or
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@ -120,9 +85,65 @@ slice is opened, its disklabel will be initialized with parameters returned
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by the drive and a single
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.Sq Li c
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partition encompassing the entire slice.
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.Sh CACHE EFFECTS
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Many direct access devices are equiped with read and/or write caches.
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Parameters affecting the device's cache are stored in mode page 8,
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the caching control page. Mode pages can be examined and modified
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via the
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.Xr camcontrol 8
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utility.
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The read cache is used to store data from device initiated read ahead
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operations as well as frequently used data. The read cache is transparant
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to the user and can be enabled without any adverse effect. Most devices
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with a read cache come from the factory with it enabled. The read cache
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can be disabled by setting the
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.Tn RCD
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(Read Cache Disable) bit in the caching control mode page.
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The write cache can greatly decrease the latency of write operations
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and allows the device to reorganize writes to increase efficiency and
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performance. This performance gain comes at a price. Should the device
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lose power while its cache contains uncommitted write operations, these
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writes will be lost. The effect of a loss of write transactions on
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a file system is non-deterministic and can cause corruption. Most
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devices age write transactions to limit vulnerability to a few transactions
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recently reported as complete, but it is none-the-less recommended that
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systems with write cache enabled devices reside on an Uninterruptible
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Power Supply (UPS). The
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.Nm da
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device driver ensures that the cache and media are synchronized upon
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final close of the device or an unexpected shutdown (panic) event. This
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ensures that it is safe to disconnect power once the operating system
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has reported that it has halted. The write cache can be enabeled by
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setting the
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.Tn WCE
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(Write Cache Enable) bit in the caching control mode page.
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.Sh TAGGED QUEUING
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The
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.Nm da
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device driver will take full advantage of the SCSI feature known as tagged
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queuing. Tagged queuing allows the device to process multiple transaction
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concurrently, often re-ordering them to reduce the number and length of
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seeks. To ensure that transactions to distant portions of the media,
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which may be deferred indefinitely by servicing requests nearer the current
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head position, are completed in a timely fashion, an ordered tagged
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transaction is sent every 15 seconds during continuous device operation.
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.Sh BAD BLOCK RECOVERY
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Direct Access devices have the capability of mapping out portions of
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defective media. Media recovery parameters are located in mode page 1,
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the Read-Write Error Recovery mode page. The most imporatant media
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remapping features are 'Auto Write Reallocation' and 'Auto Read
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Reallocation' which can be enabled via the AWRE and ARRE bits,
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respecitvely, of the Read-Write Error Recovery page.
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Many devices do not ship from the factory with these feature enabled.
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Mode pages can be examined and modified
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via the
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.Xr camcontrol 8
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utility.
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.Sh KERNEL CONFIGURATION
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It is only necessary to explicitly configure one
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.Nm
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.Nm da
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device; data structures are dynamically allocated as disks are found
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on the
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.Tn SCSI
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@ -139,11 +160,11 @@ disks as well as to other disks. They are defined in the header file
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.It Dv DIOCSBAD
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Usually used to set up a bad-block mapping system on the disk.
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.Tn SCSI
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drive incorporate their own bad-block mapping so this command is not
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drives incorporate their own bad-block mapping so this command is not
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implemented.
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.It Dv DIOCGDINFO
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Read, from the kernel, the in-core copy of the disklabel for the
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drive. This may be a fictitious disklabel if the drive has never
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drive. This may be a ficticious disklabel if the drive has never
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been initialized, in which case it will contain information read
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from the
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.Tn SCSI
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@ -161,36 +182,23 @@ Give the driver a new disklabel to use. The driver
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.Em will
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write the new disklabel to the disk.
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.El
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.Pp
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In addition, the
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.Xr scsi 4
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general
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.Fn ioctl
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commands may be used with the
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.Nm
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driver, but only against the
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.Sq Li c
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(whole disk) partition.
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.Sh NOTES
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If a removable device is attached to the
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.Nm
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driver, then the act of changing the media will invalidate the
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disklabel and information held within the kernel. To avoid
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corruption, all accesses to the device will be discarded until there
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are no more open file descriptors referencing the device. During this
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period, all new open attempts will be rejected. When no more open
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file descriptors reference the device, the first next open will load a
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new set of parameters (including disklabel) for the drive.
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If device becomes invalidated (media is removed, device becomes unresponsive)
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the disklabel and information held within the kernel about the device will
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be invalided. To avoid corruption of a newly inserted piece of media or
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a replacement device, all accesses to the device will be discarded until
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the last file descriptor referencing the old device is closed. During this
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period, all new open attempts will be rejected.
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.Sh FILES
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.Bl -tag -width /dev/rsdXXXXX -compact
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.It Pa /dev/rsd Ns Ar u
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.It Pa /dev/rda Ns Ar u
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raw mode
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.Tn SCSI
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disk unit
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.Ar u ,
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accessed as an unpartitioned device
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.Sm off
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.It Pa /dev/sd Ar u Pa s Ar n
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.It Pa /dev/da Ar u Pa s Ar n
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.Sm on
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block mode
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.Tn SCSI
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@ -200,7 +208,7 @@ slice
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.Ar n ,
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accessed as an unpartitioned device
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.Sm off
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.It Pa /dev/rsd Ar u Pa s Ar n
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.It Pa /dev/rda Ar u Pa s Ar n
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.Sm on
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raw mode
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.Tn SCSI
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@ -209,7 +217,7 @@ disk unit
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slice
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.ar n ,
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accessed as an unpartitioned device
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.It Pa /dev/sd Ns Ar u Ns Ar p
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.It Pa /dev/da Ns Ar u Ns Ar p
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block mode
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.Tn SCSI
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disk unit
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@ -218,7 +226,7 @@ first
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.Tn FreeBSD
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slice, partition
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.Ar p
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.It Pa /dev/rsd Ns Ar u Ns Ar p
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.It Pa /dev/dad Ns Ar u Ns Ar p
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raw mode
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.Tn SCSI
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disk unit
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@ -229,7 +237,7 @@ slice, partition
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.Ar p
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.Sm off
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.It Xo
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.Pa /dev/sd
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.Pa /dev/da
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.Ar u
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.Pa s
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.Ar n
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@ -245,7 +253,7 @@ slice, partition
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.Ar p
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.Sm off
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.It Xo
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.Pa /dev/rsd
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.Pa /dev/dad
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.Ar u
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.Pa s
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.Ar n
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@ -266,12 +274,17 @@ None.
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.Xr disklabel 5 ,
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.Xr disklabel 8 ,
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.Xr fdisk 8 ,
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.Xr sd 9
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.Xr da 9
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.Sh HISTORY
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The
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.Nm
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driver was originally written for
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driver was written for the
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.Tn CAM
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.Tn SCSI
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subsystem by Justin T. Gibbs.
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Many ideas were gleaned from the
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.Nm sd
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device driver written and ported from
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.Tn Mach
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2.5, and was ported to
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.Tn FreeBSD
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2.5
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by Julian Elischer. Support for slices was written by Bruce Evans.
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