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NAMEda —
SCSI Direct Access device driver
SYNOPSISdevice da
DESCRIPTIONTheda 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. CACHE EFFECTSMany 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
TAGGED QUEUINGTheda 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.
BAD BLOCK RECOVERYDirect 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.KERNEL CONFIGURATIONIt is only necessary to explicitly configure oneda
device; data structures are dynamically allocated as disks are found on the
SCSI bus.
SYSCTL VARIABLESThe following variables are available as both sysctl(8) variables and loader(8) tunables:
NOTESIf 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.FILES
DIAGNOSTICSNone.SEE ALSOada(4), cam(4), geom(4), nda(4), gpart(8)HISTORYTheda 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.
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