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AIX Version 4.3 Kernel Extensions and Device Support Programming Concepts

SCSI Command Tag Queuing

Note: This operation is not supported by all SCSI I/O controllers.

SCSI command tag queuing refers to queuing multiple commands to a SCSI device. Queuing to the SCSI device can improve performance because the device itself determines the most efficient way to order and process commands. SCSI devices that support command tag queuing can be divided into two classes: those that clear their queues on error and those that do not. Devices that do not clear their queues on error resume processing of queued commands when the error condition is cleared (typically by receiving the next command). Devices that do clear their queues flush all commands currently outstanding.

Command tag queueing requires the SCSI adapter, the SCSI device, the SCSI device driver, and the SCSI adapter driver to support this capability. For a SCSI device driver to queue multiple commands to a SCSI device (that supports command tag queuing), it must be able to provide at least one of the following values in the sc_buf.q_tag_msg : SC_SIMPLE_Q, SC_HEAD_OF_Q, or SC_ORDERED_Q. The SCSI disk device driver and SCSI adapter driver do support this capability. This implementation provides some queuing-specific changeable attributes for disks that can queue commands. With this information, the disk device driver attempts to queue to the disk, first by queuing commands to the adapter driver. The SCSI adapter driver then queues these commands to the adapter, providing that the adapter supports command tag queuing. If the SCSI adapter does not support command tag queuing, then the SCSI adapter driver sends only one command at a time to the SCSI adapter and so multiple commands are not queued to the SCSI disk.

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