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⟩