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
adadriver will retry a READ or WRITE command. This does not affect the number of retries used during probe time or for theadadriver dump routine. This value currently defaults to 4. - kern.cam.ada.default_timeout
-
This variable determines how long the
adadriver 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
HISTORY
The ada driver first appeared in
FreeBSD 8.0.
AUTHORS
Alexander Motin <mav@FreeBSD.org>