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 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.ada.write_cache

       kern.cam.ada.X.write_cache

           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
           runtime  take  effect  only  after  device reset.  The global default is currently enabled.  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), siis(4)

HISTORY

       The ada driver first appeared in FreeBSD 8.0.

AUTHORS

       Alexander Motin <mav@FreeBSD.org>

Debian                                          February 8, 2012                                          ADA(4)