da - SCSI Direct Access device driver
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 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 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 the 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 60 seconds.
This variable determines what the minimum READ/WRITE CDB size is for a given unit. (The %d above denotes the unit number of the driver instance, e.g. 1, 2, 4, 8, etc.) Valid minimum command size values are 6, 10, 12 and 16 bytes. The default is 6 bytes.
The driver issues a CAM Path Inquiry CCB at probe time to determine whether the protocol the device in question speaks (e.g. ATAPI) typically does not allow 6 byte commands. If it does not, the driver will default to using at least 10 byte CDBs. If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the 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.)
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Created 1996-2022 by Maxim Chirkov
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