freebsd-skq/share/man/man4/ada.4

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.Dd February 21, 2014
.Dt ADA 4
.Os
.Sh NAME
.Nm ada
.Nd ATA Direct Access device driver
.Sh SYNOPSIS
.Cd device ada
.Sh DESCRIPTION
The
.Nm
driver provides support for direct access devices, implementing the
.Tn ATA
command protocol, that are attached to the system through a host adapter
supported by the CAM subsystem.
.Pp
The host adapter must also be separately configured into the system before an
.Tn ATA
direct access device can be configured.
.Sh COMMAND QUEUING
Command queueing allows the device to process multiple transactions
concurrently, often re-ordering them to reduce the number and length of
seeks.
.Tn ATA
defines two types of queueing:
.Tn TCQ (Tagged Command Queueing, PATA legacy)
and
.Tn NCQ (Native Command Queueing, SATA) .
The
.Nm
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.
.Sh 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
.Xr camcontrol 8
utility.
.Pp
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.
.Pp
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
.Nm
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.
.Sh SYSCTL VARIABLES
The following variables are available as both
.Xr sysctl 8
variables and
.Xr loader 8
tunables:
.Bl -tag -width 12
.It Va kern.cam.ada.retry_count
.Pp
This variable determines how many times the
.Nm
driver will retry a READ or WRITE command.
This does not affect the number of retries used during probe time or for
the
.Nm
driver dump routine.
This value currently defaults to 4.
.It Va kern.cam.ada.default_timeout
.Pp
This variable determines how long the
.Nm
driver will wait before timing out an outstanding command.
The units for this value are seconds, and the default is currently 30
seconds.
.It Va kern.cam.ada.spindown_shutdown
.Pp
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.
.It Va kern.cam.ada.write_cache
.It Va kern.cam.ada. Ns Ar X Ns Va .write_cache
.Pp
These variables determine 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 at runtime take effect only after device reset 
.Pq using the reset subcommand of Xr camcontrol 8 .
Because of that, this setting should be changed in
.Pa /boot/loader.conf
instead of
.Pa /etc/sysctl.conf .
The global default is currently 1.
The per-device default is to leave it as-is (follow global setting).
.El
.Sh FILES
.Bl -tag -width ".Pa /dev/ada*" -compact
.It Pa /dev/ada*
ATA device nodes
.El
.Sh SEE ALSO
.Xr ahci 4 ,
.Xr cam 4 ,
.Xr da 4 ,
.Xr siis 4
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 8.0 .
.Sh AUTHORS
.An Alexander Motin Aq mav@FreeBSD.org