ada

NAME

ada — ATA Direct Access device driver

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 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).