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 queuing 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 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 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 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 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 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 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.

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 seconds, 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 currently enabled.

     kern.cam.sort_io_queue

     kern.cam.ada.X.sort_io_queue

         These variables determine whether request queue should be sorted trying to optimize head
         seeks.  Set to 1 to enable sorting, 0 to disable, -1 to leave it as-is.  The default is
         sorting enabled for HDDs and disabled SSDs.

     kern.cam.ada.read_ahead

     kern.cam.ada.X.read_ahead

     kern.cam.ada.write_cache

     kern.cam.ada.X.write_cache

         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 modified 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).

FILES

     /dev/ada*  ATA device nodes

SEE ALSO

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

HISTORY

     The ada driver first appeared in FreeBSD 8.0.

AUTHORS

     Alexander Motin <mav@FreeBSD.org>