Tagged Command Queuing

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"TCQ" redirects here. For the IATA code, see Crnl. FAP Carlos Ciriani Santa Rosa International Airport.

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^[1]. 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.

1 Overview
2 Comparison of SCSI TCQ, ATA TCQ, and SATA NCQ
3 See also
4 References
5 External links

[edit] 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.

[edit] Comparison of SCSI TCQ, ATA TCQ, and SATA NCQ

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SCSI TCQ is the first popular version of TCQ and is still popular today. The SCSI-3 protocol itself permits 64 bits to be used in the tag field, allowing up to 2⁶⁴ tasks in one task set to be issued before requiring that some of them complete before any more commands be issued^[2]. However, different protocols that implement the SCSI protocol might not permit the use of all 64 bits. 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.

TCQ allows tasks to be entered into a queue using one of three different modes: head of queue, ordered, and simple^[2]. In head of queue mode, unique to SCSI TCQ, a task is pushed into the front of a queue, ahead of all other tasks including other pending head of queue tasks^[2]. ^[3]^[1]. 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^[2]. Simple mode allows tasks to execute in any order that does not violate the constraints on the tasks in the other two modes^[2]. After a command in a task is completed, a notification is sent by the device that completed the command to the host bus adapter^[2]. 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. The older ISA bus required a SCSI host adapter to generate a interrupt to cause the CPU to program the third-party DMA engine to perform a transfer, and then required another interrupt to notify the CPU that a task in the queue was finished^[1], causing high CPU overhead.

ATA TCQ was developed to try to bring the same benefits as SCSI to ATA drives. It is available in both Parallel and Serial ATA. This effort was not very successful because the ATA bus started out as a reduced-pin-count ISA bus. 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^[1]. Since responding to interrupts causes much overhead, CPU utilization rose quickly when ATA TCQ was enabled ^[1]. Also, since interrupt service time can be unpredictable, there are times when the disk is ready to transfer data but is unable to because it must wait for the CPU to respond to the interrupt so that it knows that it needs to program the third party DMA engine^[1]. Therefore, this standard was rarely implemented because it caused high CPU utilization without improving performance enough to make this worthwhile^[1]. This standard allows up to 32 outstanding commands per device ^[3].

SATA NCQ is a new standard which does not require CPU interrupts. Like ATA TCQ, it allows up to 32 outstanding commands per device^[1], but was designed to take advantage of the ability of SATA host bus adapters that are not emulating Parallel ATA behavior to support first party DMA^[1]. Instead of interrupting the CPU before the task, the hard drive itself programs the first-party DMA engine integrated into the host bus adapter, and then the DMA engine performs the task^[1]. To further reduce the interrupt overhead, the drive can withhold the interrupt with the task completed messages until it gathers many of them to send at once, allowing the CPU to notify many threads at once that their tasks have been completed^[1]. 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^[1]. This allows the hard disk firmware design to trade off disk performance against CPU utilization by determining when to withhold and when to send completion messages^[1].