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>

Debian                                          December 20, 2017                                         ADA(4)