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This variable determines what the minimum READ/WRITE CDB size is for a given da unit. (The %d above denotes the unit number of the da 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 da 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 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.)