bionic (4) ada.4freebsd.gz

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), siis(4)

HISTORY

     The ada driver first appeared in FreeBSD 8.0.

AUTHORS

     Alexander Motin <mav@FreeBSD.org>