Disk array controller

Last updated

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

Contents

Disk array controller is often ambiguously shortened to disk controller which can also refer to the circuitry responsible for managing internal disk drive operations.

Front-end and back-end side

A disk array controller provides front-end interfaces and back-end interfaces.

A single controller may use different protocols for back-end and for front-end communication. Many enterprise controllers use FC on front-end and SATA on back-end.

Enterprise controllers

In a modern enterprise architecture disk array controllers (sometimes also called storage processors, or SPs [1] ) are parts of physically independent enclosures, such as disk arrays placed in a storage area network (SAN) or network-attached storage (NAS) servers.

Those external disk arrays are usually purchased as an integrated subsystem of RAID controllers, disk drives, power supplies, and management software. It is up to controllers to provide advanced functionality (various vendors name these differently):

Simple controllers

Promise Technology ATA RAID controller Promise Ultra33.jpg
Promise Technology ATA RAID controller

A simple disk array controller may fit inside a computer, either as a PCI/PCIe expansion card or just built onto a motherboard. Such a controller usually provides host bus adapter (HBA) functionality itself to save physical space. Hence it is sometimes called a RAID adapter.

As of February  2007 Intel started integrating their own Matrix RAID controller in their more upmarket motherboards, giving control over 4 devices and an additional 2 SATA connectors, and totalling 6 SATA connections (3 Gbit/s each). For backward compatibility one IDE connector able to connect 2 ATA devices (100 Mbit/s) is also present.

History

While hardware RAID controllers have been available for a long time, they initially required expensive Parallel SCSI hard drives and aimed at the server and high-end computing market. SCSI technology advantages include allowing up to 15 devices on one bus, independent data transfers, hot-swapping, much higher MTBF.

Around 1997, with the introduction of ATAPI-4 (and thus Ultra-DMA-Mode, which enabled fast data transfers with less CPU utilization) the first ATA RAID controllers were introduced as PCI expansion cards. Those RAID systems made their way to the consumer market, for users wanting the fault-tolerance of RAID without investing in expensive SCSI drives.

Fast consumer drives make it possible to build RAID systems at lower cost than with SCSI, but most ATA RAID controllers lack a dedicated buffer or high-performance XOR hardware for parity calculation. As a result, ATA RAID performs relatively poorly compared to most SCSI RAID controllers. Additionally, data safety suffers if there is no battery backup to finish writes interrupted by a power outage.

OS support

Because the hardware RAID controllers present assembled RAID volumes, operating systems aren't strictly required to implement the complete configuration and assembly for each controller. Very often only the basic features are implemented in the open-source software driver, with extended features being provided through binary blobs directly by the hardware manufacturer.

Normally, RAID controllers can be fully configured through card BIOS before an operating system is booted, and after the operating system is booted, proprietary configuration utilities are available from the manufacturer of each controller, because the exact feature set of each controller may be specific to each manufacturer and product. Unlike the network interface controllers for Ethernet, which can be usually be configured and serviced entirely through the common operating system paradigms like ifconfig in Unix, without a need for any third-party tools, each manufacturer of each RAID controller usually provides their own proprietary software tooling for each operating system that they deem to support, ensuring a vendor lock-in, and contributing to reliability issues. [2]

For example, in FreeBSD, in order to access the configuration of Adaptec RAID controllers, users are required to enable Linux compatibility layer, and use the Linux tooling from Adaptec, [3] potentially compromising the stability, reliability and security of their setup, especially when taking the long term view in mind. [2] However, this greatly depends on the controller, and whether appropriate hardware documentation is available in order to write a driver, and some controllers do have open-source versions of their configuration utilities, for example, mfiutil and mptutil is available for FreeBSD since FreeBSD 8.0 (2009), [4] [5] as well as mpsutil/mprutil since 2015, [6] each supporting only their respective device drivers, this latter fact contributing to code bloat.

Some other operating systems have implemented their own generic frameworks for interfacing with any RAID controller, and provide tools for monitoring RAID volume status, as well as facilitation of drive identification through LED blinking, alarm management, hot spare disk designations and data scrubbing § RAID from within the operating system without having to reboot into card BIOS. For example, this was the approach taken by OpenBSD in 2005 with its bio(4) pseudo-device driver and the bioctl utility, which provide volume status, and allow LED/alarm/hotspare control, as well as the sensors (including the drive sensor) for health monitoring; [7] this approach has subsequently been adopted and extended by NetBSD in 2007 as well. [8]

With bioctl, the feature set is intentionally kept to a minimum, so that each controller can be supported by the tool in the same way; the initial configuration of the controller is meant to be performed through card BIOS, [7] but after the initial configuration, all day-to-day monitoring and repair should be possible with unified and generic tools, which is what bioctl is set to accomplish.

Related Research Articles

<span class="mw-page-title-main">Parallel ATA</span> Computer storage interface standard

Parallel ATA (PATA), originally AT Attachment, also known as Integrated Drive Electronics (IDE), is a 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, optical disc drives, and tape drives in computers.

<span class="mw-page-title-main">SCSI</span> 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, best known for its use with storage devices such as hard disk drives. SCSI was introduced in the 1980s and has seen widespread use on servers and high-end workstations, with new SCSI standards being published as recently as SAS-4 in 2017.

RAID is a data storage virtualization technology that combines multiple physical data storage components into one or more logical units for the purposes of data redundancy, performance improvement, or both. This is in contrast to the previous concept of highly reliable mainframe disk drives referred to as "single large expensive disk" (SLED).

Internet Small Computer Systems Interface or iSCSI is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP network. iSCSI facilitates data transfers over intranets and to manage storage over long distances. It can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval.

<span class="mw-page-title-main">SATA</span> Computer bus interface for storage devices

SATA 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.

<span class="mw-page-title-main">Host adapter</span> Computer hardware device

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, SAS, NVMe, Fibre Channel and SATA devices. Devices for connecting to FireWire, USB and other devices may also be called host controllers or host adapters.

<span class="mw-page-title-main">Native Command Queuing</span>

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.

The Advanced Host Controller Interface (AHCI) is a technical standard defined by Intel that specifies the register-level interface of Serial ATA (SATA) host controllers in a non-implementation-specific manner in its motherboard chipsets.

<span class="mw-page-title-main">USB mass storage device class</span> USB device class for drives

The USB mass storage device class is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.

Data scrubbing is an error correction technique that uses a background task to periodically inspect main memory or storage for errors, then corrects detected errors using redundant data in the form of different checksums or copies of data. Data scrubbing reduces the likelihood that single correctable errors will accumulate, leading to reduced risks of uncorrectable errors.

Vinum is a logical volume manager, also called software RAID, allowing implementations of the RAID-0, RAID-1 and RAID-5 models, both individually and in combination. The original Vinum was part of the base distribution of the FreeBSD operating system since 3.0, and also NetBSD between 2003-10-10 and 2006-02-25, as well as descendants of FreeBSD, including DragonFly BSD; in more recent versions of FreeBSD, it has been replaced with gvinum, which was first introduced around FreeBSD 6. Vinum source code is maintained in the FreeBSD and DragonFly source trees. Vinum supports RAID levels 0, 1, 5, and JBOD. Vinum was inspired by Veritas Volume Manager.

In computing, error recovery control (ERC) is a feature of hard disks which allow a system administrator to configure the amount of time a drive's firmware is allowed to spend recovering from a read or write error. Limiting the recovery time allows for improved error handling in hardware or software RAID environments. In some cases, there is a conflict as to whether error handling should be undertaken by the hard drive or by the RAID implementation, which leads to drives being marked as unusable and significant performance degradation, when this could otherwise have been avoided.

<span class="mw-page-title-main">ATTO Technology</span> Storage connectivity product manufacturer

ATTO Technology, Inc. is a manufacturer of storage connectivity products for data-intensive computing. ATTO manufactures Fibre Channel and SAS/SATA host bus adapters, RAID adapters, Fibre Channel switches, protocol conversion bridges, storage controllers, MacOS iSCSI initiator software and acceleration software with storage interface connectivity to SATA, SAS, Fibre Channel, Thunderbolt devices, Ethernet and NVMe.

<span class="mw-page-title-main">Storage area network</span> 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).

A trim command allows an operating system to inform a solid-state drive (SSD) which blocks of data are no longer considered to be "in use" and therefore can be erased internally.

NVM Express (NVMe) or Non-Volatile Memory Host Controller Interface Specification (NVMHCIS) is an open, logical-device interface specification for accessing a computer's non-volatile storage media usually attached via the PCI Express bus. The initial NVM stands for non-volatile memory, which is often NAND flash memory that comes in several physical form factors, including solid-state drives (SSDs), PCIe add-in cards, and M.2 cards, the successor to mSATA cards. NVM Express, as a logical-device interface, has been designed to capitalize on the low latency and internal parallelism of solid-state storage devices.

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.

Enterprise Storage OS, also known as ESOS, is a Linux distribution that serves as a block-level storage server in a storage area network (SAN). ESOS is composed of open-source software projects that are required for a Linux distribution and several proprietary build and install time options. The SCST project is the core component of ESOS; it provides the back-end storage functionality.

The bio(4) pseudo-device driver and the bioctl(8) utility implement a generic RAID volume management interface in OpenBSD and NetBSD. The idea behind this software is similar to ifconfig, where a single utility from the operating system can be used to control any RAID controller using a generic interface, instead of having to rely on many proprietary and custom RAID management utilities specific for each given hardware RAID manufacturer. Features include monitoring of the health status of the arrays, controlling identification through blinking the LEDs and managing of sound alarms, and specifying hot spare disks. Additionally, the softraid configuration in OpenBSD is delegated to bioctl as well; whereas the initial creation of volumes and configuration of hardware RAID is left to card BIOS as non-essential after the operating system has already been booted. Interfacing between the kernel and userland is performed through the ioctl system call through the /dev/bio pseudo-device.

<span class="mw-page-title-main">Future Domain</span> American computer hardware company

Future Domain Corporation was a privately held American computer hardware company active from 1982 to 1995 and based in Orange County, California. The company was among the first to produce Small Computer System Interface (SCSI) device controller expansion cards, later controller ICs. It was acquired by Adaptec in 1995 for US$25 million.

References

  1. ↑ "Storage Basics - Part V: Controllers, Cache and Coalescing". 23 March 2010.
  2. 1 2 "3.8: "Hackers of the Lost RAID"". OpenBSD Release Songs. OpenBSD. 2005-11-01. Retrieved 2019-03-23.
  3. ↑ Scott Long; Adaptec, Inc (2000). "aac(4) — Adaptec AdvancedRAID Controller driver". BSD Cross Reference. FreeBSD.
  4. ↑ "mfiutil — Utility for managing LSI MegaRAID SAS controllers". BSD Cross Reference. FreeBSD.
  5. ↑ "mptutil — Utility for managing LSI Fusion-MPT controllers". BSD Cross Reference. FreeBSD.
  6. ↑ "mpsutil — Utility for managing LSI Fusion-MPT 2/3 controllers". BSD Cross Reference. FreeBSD.
  7. 1 2 Theo de Raadt (2005-09-09). "RAID management support coming in OpenBSD 3.8". misc@ (Mailing list). OpenBSD.
  8. ↑ Constantine A. Murenin (2010-05-21). "1.1. Motivation; 4. Sensor Drivers; 7.1. NetBSD envsys / sysmon". OpenBSD Hardware Sensors — Environmental Monitoring and Fan Control (MMath thesis). University of Waterloo: UWSpace. hdl:10012/5234. Document ID: ab71498b6b1a60ff817b29d56997a418.
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.