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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.

FILES

     /dev/ada*  ATA device nodes

SEE ALSO

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

HISTORY

     The ada driver first appeared in FreeBSD 8.0.

AUTHORS

     Alexander Motin <mav@FreeBSD.org>