Tagged Command Queuing

Last updated

Tagged Command Queuing (TCQ) is a technology built into certain ATA and SCSI [1] 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. [2] SCSI TCQ does not suffer from the same limitations as ATA TCQ.

Contents

Without TCQ, an operating system was limited to sending one request at a time. To boost performance, the OS had to determine the order of the requests based on its own – possibly incorrect – perspective of the hard drive activity (otherwise known as I/O scheduling). 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). Thus TCQ can improve the overall performance of a hard drive if it is implemented correctly.

Overview

For increased efficiency the sectors should be serviced in order of proximity to the current head position, not the order received. The queue is constantly receiving new requests, 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 may dramatically increase performance in a system with many users making widely varied requests on the disk surface.

Comparison of SCSI TCQ, ATA TCQ, and SATA NCQ

SCSI TCQ

SCSI TCQ was the first popular version of TCQ and is still popular today. It allows tasks to be entered into a queue using one of three different modes: [3]

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] [4] This mode is not used much because it can cause resource starvation when abused.

In ordered mode, a task must execute after all older tasks have completed and before all newer tasks begin to execute (excluding newer head of queue tasks). [3]

Simple mode allows tasks to execute in any order that does not violate the constraints on the tasks in the other two modes. [3] After a command in a task is completed, a notification is sent by the device that completed the command to the host bus adapter. [3]

Whether or not SCSI TCQ causes massive interrupt overhead depends on the bus being used to connect the SCSI host bus adapter. On Conventional 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 an 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, [2] causing high CPU overhead.

SCSI TCQ Tag Length

The SCSI-3 protocol 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. [3] 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.

ATA TCQ

ATA TCQ was developed in attempt 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. [2] Since responding to interrupts uses CPU time, CPU utilization rose quickly when ATA TCQ was enabled. [2] Also, since interrupt service time can be unpredictable, there are times when the disk is ready to transfer data but is unable to do so because it must wait for a CPU to respond to the interrupt so that the CPU knows that it needs to program the third party DMA engine. [2]

Therefore, this standard was rarely implemented because it caused high CPU utilization without improving performance enough to make this worthwhile. [2] This standard allows up to 32 outstanding commands per device. [4]

SATA NCQ

SATA NCQ is a modern standard which drastically reduces the number of required CPU interrupts compared to ATA TCQ. Like ATA TCQ, it allows up to 32 outstanding commands per device, [2] 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. [2] 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 execute the command. [2] 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 operating system to notify many threads simultaneously that their tasks have been completed. [2] If another task completes after such an interrupt is sent, the host bus adapter can concatenate the completion messages together if the first set of completion messages has not been sent to the CPU. [2] 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. [2]

Related Research Articles

Parallel ATA Interface standard for the connection of storage devices

Parallel ATA (PATA), originally AT Attachment, also known as ATA or IDE is standard interface designed for IBM PC-compatible computers. It was first developed by Western Digital and Compaq in 1986 for compatible hard drives and CD or DVD drives. The connection is used for storage devices such as hard disk drives, floppy disk drives, and optical disc drives in computers.

Bus (computing) System that transfers data between components within a computer

In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components and software, including communication protocols.

Industry Standard Architecture 16-bit internal bus

Industry Standard Architecture (ISA) is the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.

SCSI Set of computer and peripheral connection standards

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interfaces. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. The initial Parallel SCSI was most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices.

Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).

Serial ATA Computer bus interface for storage devices

Serial ATA is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to become the predominant interface for storage devices.

Disk controller

The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus connecting it to the rest of the system.

Host adapter

In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other network and storage devices. The terms are primarily used to refer to devices for connecting SCSI, Fibre Channel and SATA devices. Devices for connecting to IDE, Ethernet, FireWire, USB and other systems may also be called host adapters.

A disk array controller is a device that manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.

Native Command Queuing

In computing, Native Command Queuing (NCQ) is an extension of the Serial ATA protocol allowing hard disk drives to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary drive head movement, resulting in increased performance for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications.

Serial Attached SCSI Point-to-point serial protocol for enterprise storage

In computing, Serial Attached SCSI (SAS) is a point-to-point serial protocol that moves data to and from computer-storage devices such as hard disk drives and tape drives. SAS replaces the older Parallel SCSI bus technology that first appeared in the mid-1980s. SAS, like its predecessor, uses the standard SCSI command set. SAS offers optional compatibility with Serial ATA (SATA), versions 2 and later. This allows the connection of SATA drives to most SAS backplanes or controllers. The reverse, connecting SAS drives to SATA backplanes, is not possible.

BIOS interrupt calls are a facility that operating systems and application programs use to invoke the facilities of the Basic Input/Output System software on IBM PC compatible computers. Traditionally, BIOS calls are mainly used by DOS programs and some other software such as boot loaders. BIOS runs in the real address mode of the x86 CPU, so programs that call BIOS either must also run in real mode or must switch from protected mode to real mode before calling BIOS and then switching back again. For this reason, modern operating systems that use the CPU in Protected mode or Long mode generally do not use the BIOS interrupt calls to support system functions, although they use the BIOS interrupt calls to probe and initialize hardware during booting. Real mode has the 1MB memory limitation, modern boot loaders use the unreal mode or protected mode to access up to 4GB memory.

QEMU Free virtualization and emulation software

QEMU is a free and open-source emulator. It emulates the machine's processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it to run a variety of guest operating systems. It can interoperate with Kernel-based Virtual Machine (KVM) to run virtual machines at near-native speed. QEMU can also do emulation for user-level processes, allowing applications compiled for one architecture to run on another.

A SCSI connector is used to connect computer parts that use a system called SCSI to communicate with each other. Generally, two connectors, designated male and female, plug together to form a connection which allows two components, such as a computer and a disk drive, to communicate with each other. SCSI connectors can be electrical connectors or optical connectors. There have been a large variety of SCSI connectors in use at one time or another in the computer industry. Twenty-five years of evolution and three major revisions of the standards resulted in requirements for Parallel SCSI connectors that could handle an 8, 16 or 32 bit wide bus running at 5, 10 or 20 megatransfer/s, with conventional or differential signaling. Serial SCSI added another three transport types, each with one or more connector types. Manufacturers have frequently chosen connectors based on factors of size, cost, or convenience at the expense of compatibility.

Parallel SCSI

Parallel SCSI is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it. Both ends of the bus must be terminated.

In computer science, a command queue is a queue for enabling the delay of command execution, either in order of priority, on a first-in first-out basis, or in any order that serves the current purpose. Instead of waiting for each command to be executed before sending the next one, the program just puts all the commands in the queue and goes on doing other things while the queue is processed by the operating system.

sync is a standard system call in the Unix operating system, which commits all data in the kernel filesystem to non-volatile storage buffers, i.e., data which has been scheduled for writing via low-level I/O system calls. Higher-level I/O layers such as stdio may maintain separate buffers of their own.

Disk buffer

In computer storage, disk buffer is the embedded memory in a hard disk drive (HDD) acting as a buffer between the rest of the computer and the physical hard disk platter that is used for storage. Modern hard disk drives come with 8 to 256 MiB of such memory, and solid-state drives come with up to 4 GB of cache memory.

Storage area network Network which provides access to consolidated, block-level data storage

A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from servers so that the devices appear to the operating system as direct-attached storage. A SAN typically is a dedicated network of storage devices not accessible through the local area network (LAN).

Hard disk drives are accessed over one of a number of bus types, including parallel ATA, Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses with which they cannot communicate natively, such as IEEE 1394, USB, SCSI, NVMe and Thunderbolt.

References

  1. ↑ in the form of Parallel SCSI, Serial attached SCSI, and Fibre Channel drives
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 Dees, Brian (November–December 2005). "Native command queuing - advanced performance in desktop storage". IEEE Potentials. 24 (4): 4–7. doi:10.1109/MP.2005.1549750. S2CID   36264057.
  3. 1 2 3 4 5 6 "SCSI Architecture Model - 3 (SAM-3)" (PDF). Archived from the original (PDF) on 2012-03-17. Retrieved 2007-02-24.
  4. 1 2 "1532D: AT Attachment with Packet Interface - 7 Volume 1" (PDF). 1532D: AT Attachment with Packet Interface - 7. Retrieved 2007-01-02.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.