Tagged Com
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Tagged Command Queuing (TCQ) is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive.
ATA TCQ is not identical in function to the more efficient native command queuing (NCQ) used by SATA drives. SCSI TCQ does not suffer from the same limitations as ATA TCQ.
Before TCQ, an operating system was only able to send one request at a time.
In order to boost performance, it had to decide the order of the requests based on its own, possibly incorrect, idea of what the hard drive was doing. With TCQ, the drive can make its own decisions about how to order the requests (and in turn relieve the operating system from having to do so).
The result is that TCQ can improve the overall performance of a hard drive if it is implemented correctly.
Overview
For efficiency the sectors should be serviced in order of proximity to the current head position, rather than in the order received. The queue is constantly receiving new requests and fulfilling and removing existing requests, and re-ordering the queue according to the current pending read/write requests and the changing position of the head.
The exact reordering algorithm may depend upon the controller and the drive itself, but the host computer simply makes requests as needed, leaving the controller to handle the details.
This queuing mechanism is sometimes referred to as "elevator seeking", as the image of a modern elevator in a building servicing multiple calls and processing them to minimise travel illustrates the idea well.
If the buttons for floors 5, 2, and 4 are pressed in that order with the elevator starting on floor 1, an old elevator would go to the floors in the order requested. A modern elevator processes the requests to stop at floors in the logical order 2, 4, and 5, without unnecessary travel.
Non-queueing disk drives service the requests in the order received, like an old elevator; queueing drives service requests in the most efficient order. This may improve performance slightly in a system used by a single user, but really comes into its own in a system with many users, making requests all over the disk surface.
Comparison of SCSI TCQ, ATA TCQ, and SATA NCQ
SCSI TCQ
SCSI TCQ is the first popular version of TCQ and is still popular today.
This mode is not used much because it can cause data starvation when abused.
In ordered mode, a task must execute after all older tasks and before all newer tasks excluding newer head of queue tasks.
Simple mode allows tasks to execute in any order that does not violate the constraints on the tasks in the other two modes. Whether or not SCSI TCQ causes massive interrupt overhead depends on the bus being used to connect the SCSI host bus adapter.
On PCI, PCI-X, PCI Express, and other buses that permit it, first party DMA allows for low interrupt overhead. For example, older parallel SCSI permits 8 bits of tag bits, iSCSI permits up to 32 tag bits, and Fibre Channel permits up to 16 bits of tag with tag 0xFFFF reserved.
This flexibility allows the designer of a protocol to trade off queuing ability against cost. Networks that can be large, such as iSCSI networks, benefit from more tag bits to deal with the larger number of disks in the network and the larger latencies such large networks generate, while smaller-scale networks, such as parallel SCSI chains, do not have enough disks or latency to need many tag bits and can save money by using a system supporting fewer bits.
ATA TCQ
ATA TCQ was developed to try to bring the same benefits as SCSI to ATA drives.
The requirement for software compatibility made ATA host bus adapters act like ISA bus devices without first party DMA. When a drive was ready for a transfer, it had to interrupt the CPU, wait for the CPU to ask the disk what command was ready to execute, respond with the command that it was ready to execute, wait for the CPU to program the host bus adapter's third-party DMA engine based on the result of that command, wait for the third-party DMA engine to execute the command, and then had to interrupt the CPU again to notify it when the DMA engine finished the task so that the CPU could notify the thread that requested the task that the requested task was finished.
Since responding to interrupts causes much overhead, CPU utilization rose quickly when ATA TCQ was enabled . Instead of interrupting the CPU before the task to force it to program the host bus adapter's DMA engine, the hard drive tells the host bus adapter which command it wants to execute, causing the host bus adapter to program its integrated first-party DMA engine with the parameters that were included in the command that was selected by the hard drive when it was first issued, and then the DMA engine moves the data needed to complete the command.
If another task completes after such an interrupt is sent, the host bus adapter can amend the completion messages if they have not been sent to the CPU.