A SCSI Host adapter must also be separately configured into the system before a SCSI direct access device can be configured.
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 read cache can be disabled by setting the RCD (Read Cache Disable) bit in the caching control mode page.
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 vulnerability to a few transactions recently reported as complete, but it is none-the-less recommended that systems with write cache enabled devices reside on an Uninterruptible Power Supply (UPS). The da 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 write cache can be enabled by setting the WCE (Write Cache Enable) bit in the caching control mode page.
|This variable determines how long the da driver will wait before timing out an outstanding command. The units for this value are seconds, and the default is currently 60 seconds.
|Disable detection of write-protected disks. Default is disabled. ( detection of write-protected disks is enabled ).
|Enable BIO_SPEEDUP processing. Default is enabled.
|Use UMA for CCBs. Default is enabled.
|Media polling period in seconds. Default is 3 seconds.
|This variable determines how many times the da driver will retry a READ or WRITE command. This does not affect the number of retries used during probe time or for the da driver dump routine. This value currently defaults to 4.
|Send Ordered Tags. On shutdown, step through all the da peripheral drivers, and if the device is still open, sync the disk to physical media. Default is enabled.
|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 for SSDs.
|This variable specifies method to handle BIO_DELETE requests:
|ATA TRIM via ATA COMMAND PASS THROUGH command,
|WRITE SAME(16) command with UNMAP flag,
|WRITE SAME(10) command with UNMAP flag,
|WRITE SAME(10) command without UNMAP flag,
|disable BIO_DELETE support.
This variable determines what the minimum READ/WRITE CDB size is for a
Valid minimum command size values are 6, 10, 12 and 16 bytes.
The default is 6 bytes.
The da driver issues a CAM Path Inquiry CCB at probe time to determine whether the protocol the device in question speaks (e.g.amp; ATAPI) typically does not allow 6 byte commands. If it does not, the da driver will default to using at least 10 byte CDBs. If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the da driver will then increase the default CDB size for the device to 10 bytes and retry the command. CDB size is always chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum command size, and the LBA and length of the READ or WRITE in question. (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
|SCSI disk device nodes
|September 10, 2022
|Computer science would have progressed much further and faster if all of the time and effort that has been spent maintaining and nurturing Unix had been spent on a sounder operating system.
|— The Unix Haters' handbook