What Is RAID?

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What Is RAID?

Unread postby rajeevmahura » Thu Aug 14, 2008 11:15 am

What Is RAID?
RAID (Redundant Array of Independent Disks) is a technology for managing how data is stored on the physical or hard disks that reside in your system or are attached to it. A key aspect of RAID is the ability to span hard disks so that the combined capacity of multiple disks can be treated as a single, extended chunk of disk space. Another key aspect of RAID is the ability to maintain redundant data which can be used to restore data in the event of a disk failure. RAID uses different techniques, such as striping, mirroring, and parity, to store and reconstruct data. There are different RAID levels that use different methods for storing and reconstructing data. The RAID levels have different characteristics in terms of read/write performance, data protection, and storage capacity. Not all RAID levels maintain redundant data, which means for some RAID levels lost data cannot be restored. Which RAID level you choose depends on whether your priority is performance, protection, or storage capacity.

The RAID Advisory Board (RAB) defines the specifications used to implement RAID. Although the RAID Advisory Board (RAB) defines the RAID levels, commercial implementation of RAID levels by different vendors may vary from the actual RAID specifications. An implementation used by a particular vendor may affect the read and write performance and the degree of data redundancy.

Hardware and Software RAID
RAID can be implemented with either hardware or software. A system using hardware RAID has a RAID controller that implements the RAID levels and processes data reads and writes to the array disks. When using software RAID, the operating system must implement the RAID levels. For this reason, using software RAID by itself can slow system performance. You can, however, use software RAID on top of hardware RAID volumes to provide greater performance and variety in the configuration of RAID volumes. For example, you can mirror a pair of hardware RAID 5 volumes across two RAID controllers to provide RAID controller redundancy.

This release of Storage Management only supports hardware RAID.

RAID Concepts
RAID uses particular techniques for writing data to disks. These techniques enable RAID to provide data redundancy or better performance. These techniques include:

Mirroring – Duplicating data from one disk to another disk. Mirroring provides data redundancy by maintaining two copies of the same data on different disks. If one of the disks in the mirror fails, the system can continue to operate using the unaffected disk. Both sides of the mirror contain the same data at all times. Either drive can act as the operational drive. A mirrored RAID group is comparable in performance to a RAID 5 group in read operations but faster in write operations.
Striping – Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual disk data addresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern. For example, if the virtual disk includes five physical disks, the stripe writes data to physical disks one through five without repeating any of the physical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion of a stripe that resides on a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in combination with parity does provide data redundancy.
Stripe size – The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that contains 64 KB of disk space and has 16 KB of data residing on each disk in the stripe. In this case, the stripe size is 64 KB and the stripe element size is 16 KB.
Stripe element – A stripe element is the portion of a stripe that resides on a single physical disk.
Stripe element size – The amount of disk space consumed by a stripe element. For example, consider a stripe that contains 64 KB of disk space and has 16 KB of data residing on each disk in the stripe. In this case, the stripe element size is 16 KB and the stripe size is 64 KB.
Parity – Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the striped disks fails, the data can be reconstructed from the parity information using the algorithm.
Span – A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10 or 50 virtual disk.
Disk Space and Storage Terminology
Most operating systems (OS) do not read and write data directly from the disks, but instead send read and write instructions to a controller. The controller (or adapter) is the hardware in your system that interacts directly with the hard disks to write and retrieve data. A system can have more than one controller, with each controller attached to its own set of hard disks. A controller can span the boundaries of the hard disks so as to create an extended amount of storage space using the capacity of more than one hard disk. The operating system views these spanned disks as if they are a single, large disk or drive. For example, when mapping a drive letter in MicrosoftWindows®, the storage space being mapped is usually spanned drives or a volume.

Because different hardware and software components interact with stored data, the terminology used to describe hard disks and spanned disk space varies quite a bit. For example, the terminology used by the operating system, a software application, and the controller BIOS can be completely different. The following describes commonly used terms that you will find in Storage Management and other storage products and documentation.

Hard Disk – Refers to the physical disk attached to a controller that can be manually added to or removed from the computer.
Drive – Refers to readable or writable storage space. A drive can be a hard disk, a storage device such as tape or CDROM, or spanned storage such as a volume mapped to a drive letter in Windows.
Volume – Refers to spanned disk space which the operating system sees as a single drive. This may include RAID volumes, primary and extended partitions, and logical drives associated with extended partitions. In Windows, drive letters are mapped to volumes. The term volume generally refers to the OS's view of spanned storage.
Virtual Disk (LUN, or Container) – Refers to spanned storage created by a RAID controller from one or more array disks. Although a virtual disk may be created from several array disks, it is seen by the operating system as a single disk. In order to implement RAID functions, RAID controllers must create a virtual disk. The terms LUN (Logical Unit Number) and container refer to the same concept and are more commonly used by the controller BIOS. The term virtual disk is a RAID term used by Storage Management. This term may also be found in the controller BIOS and other applications.
Array – Refers to a set of hard disks connected to a RAID controller from which virtual disks may be created. In some contexts, the term an array or array group may refer to a virtual disk that has a RAID level applied to it.
Array Disk – Refers to a disk attached to a RAID controller. Referring to a disk as an array disk indicates that the disk has the potential to be included in a virtual disk.
Layout or RAID layout – These terms refer to the RAID level.
Organizing Data Storage for Availability and Performance
RAID provides different methods or RAID levels for organizing the disk storage. Some RAID levels maintain redundant data so that you can restore data after a disk failure. Different RAID levels may also entail an increase or decrease in the system's I/O (read and write) performance.

When choosing concatenation or a RAID level, the following performance and cost considerations apply:

Availability or fault-tolerance – Availability or fault-tolerance refers to a system's ability to maintain operations and provide access to data even when one of its components has failed. In RAID volumes, availability or fault–tolerance is achieved by maintaining redundant data. Redundant data includes mirrors (duplicate data) and parity information (reconstructing data using an algorithm).
Performance – Read and write performance can be increased or decreased depending on the RAID level you choose. Some RAID levels may be more appropriate for particular applications.
Cost efficiency – Maintaining the redundant data or parity information associated with RAID volumes requires additional disk space. In situations where the data is temporary, easily reproduced, or non-essential, the increased cost of data redundancy may not be justified.
Mean Time Between Failure (MBTF) – Using additional disks to maintain data redundancy also increases the chance of disk failure at any given moment. Although this cannot be avoided in situations where redundant data is a requirement, it does have implications for the workload of your organization's system support staff.
Rajeev Mahura
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