ADA(4)							   BSD Kernel Interfaces Manual 						    ADA(4)

NAME

     ada -- ATA Direct Access device driver

SYNOPSIS

     device ada

DESCRIPTION

     The ada driver provides support for direct access devices, implementing the ATA command protocol, that are attached to the system through a
     host adapter supported by the CAM subsystem.

     The host adapter must also be separately configured into the system before an ATA direct access device can be configured.

COMMAND QUEUING

     Command queueing allows the device to process multiple transactions concurrently, often re-ordering them to reduce the number and length of
     seeks.  ATA defines two types of queueing: TCQ (Tagged Command Queueing, PATA legacy) and NCQ (Native Command Queueing, SATA).  The ada
     device driver takes full advantage of NCQ, when supported.  To ensure that transactions to distant parts of the media, which may be deferred
     indefinitely by servicing requests closer to the current head position, are completed in a timely fashion, an ordered transaction is sent
     every 7 seconds during continuous device operation.

CACHE EFFECTS

     Many direct access devices are equipped with read and/or write caches.  Parameters affecting the device's cache are reported in device IDEN-
     TIFY data and 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 trans-
     parent 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 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 the vulnerability to a few transactions recently reported as complete, but it is nonetheless recom-
     mended that systems with write cache enabled devices reside on an Uninterruptible Power Supply (UPS).  The ada 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.

SYSCTL VARIABLES

     The following variables are available as both sysctl(8) variables and loader(8) tunables:

     kern.cam.ada.retry_count

	 This variable determines how many times the ada driver will retry a READ or WRITE command.  This does not affect the number of retries
	 used during probe time or for the ada driver dump routine.  This value currently defaults to 4.

     kern.cam.ada.default_timeout

	 This variable determines how long the ada driver will wait before timing out an outstanding command.  The units for this value are sec-
	 onds, and the default is currently 30 seconds.

     kern.cam.ada.spindown_shutdown

	 This variable determines whether to spin-down disks when shutting down.  Set to 1 to enable spin-down, 0 to disable.  The default is cur-
	 rently enabled.

FILES

     /dev/ada*	ATA device nodes

SEE ALSO

     ad(4), ahci(4), cam(4), da(4), siis(4)

HISTORY

     The ada driver first appeared in FreeBSD 8.0.

AUTHORS

     Alexander Motin <mav@FreeBSD.org>

BSD
								 October 24, 2010							       BSD