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ADA(4)		       FreeBSD Kernel Interfaces Manual			ADA(4)

     ada -- ATA	Direct Access device driver

     device ada

     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 queueing allows the device	to process multiple transactions con-
     currently,	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 indefi-
     nitely 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.

     Many direct access	devices	are equipped with read and/or write caches.
     Parameters	affecting the device's cache are reported in device IDENTIFY
     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 transpar-
     ent 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 perfor-
     mance.  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 sys-
     tem 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 recommended 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.

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

	 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.

	 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.

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

	 These variables determines whether device write cache should be
	 enabled globally or per-device	or disabled.  Set to 1 to enable write
	 cache,	0 to disable, -1 to leave it as-is.  Values modified in	run-
	 time take effect only after device reset.  The	global default is cur-
	 rently	enabled.  The per-device default is to leave it	as-is (follow
	 global	setting).

     /dev/ada*	ATA device nodes

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

     The ada driver first appeared in FreeBSD 8.0.

     Alexander Motin <>

FreeBSD	9.1		       February	8, 2012			   FreeBSD 9.1


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