Manual Reference Pages - ADA (4)
- ATA Direct Access device driver
.Cd device ada
driver provides support for direct access devices, implementing the
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
direct access device can be configured.
Command queuing allows the device to process multiple transactions
concurrently, often re-ordering them to reduce the number and length of
defines two types of queuing:
TCQ (Tagged Command Queuing, PATA legacy)
NCQ (Native Command Queuing, SATA).
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.
Many direct access devices are equipped with read and/or write caches.
Parameters affecting the devices cache are reported in device IDENTIFY data
and can be examined and modified via the
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.
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
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.
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).
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.
The following variables are available as both
This variable determines how many times the
driver will retry a READ or WRITE command.
This does not affect the number of retries used during probe time or for
driver dump routine.
This value currently defaults to 4.
This variable determines how long the
driver will wait before timing out an outstanding command.
The units for this value are seconds, and the default is currently 30
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. 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. X .read_ahead
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
The global default is currently 1.
The per-device default is to leave it as-is (follow global setting).
This variable determines whether
symbolic links are created,
which tries to mimic old
Set to 1 to enable legacy aliases symlinks, 0 to disable.
The default is currently enabled.
ATA device nodes
driver first appeared in
.Fx 8.0 .
.An Alexander Motin Aq mav@FreeBSD.org
Visit the GSP FreeBSD Man Page Interface.
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