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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 queuing 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 queuing: TCQ (Tagged Command Queuing,
     PATA legacy) and NCQ (Native Command Queuing, 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 determine whether request queue should	be sorted try-
	 ing to	optimize head seeks.  Set to 1 to enable sorting, 0 to dis-
	 able, -1 to leave it as-is.  The default is sorting enabled for HDDs
	 and disabled SSDs.

	 These variables determine whether device read-ahead and write caches
	 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 modi-
	 fied at runtime take effect only after	device reset (using the	reset
	 subcommand of camcontrol(8)).	Because	of that, this setting should
	 be changed in /boot/loader.conf instead of /etc/sysctl.conf.  The
	 global	default	is currently 1.	 The per-device	default	is to leave it
	 as-is (follow global setting).

     /dev/ada*	ATA device nodes

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

     The ada driver first appeared in FreeBSD 8.0.

     Alexander Motin <>

FreeBSD	11.1			 June 19, 2015			  FreeBSD 11.1


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