RAID — which stands for Redundant Array of Inexpensive Disks [1], or alternatively Redundant Array of Independent Disks (a less specific name, and thus now the generally accepted one[2]) — is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance, reliability, and/or larger data volume sizes.
The phrase "RAID" is an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. RAID's various designs all involve two key design goals: increased data reliability and increased input/output performance. When several physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across several disks, but the array is seen by the computer user and operating system as one single disk. RAID can be set up to serve several different purposes.
Purpose and basics
Some arrays are "redundant" in a way that writes extra data derived from the original data across the array organized so that the failure of one (sometimes more) disks in the array will not result in loss of data; the bad disk is replaced by a new one, and the data on it reconstructed from the remaining data and the extra data. A redundant array allows less data to be stored. For instance, a 2-disk RAID 1 array loses half of its capacity, and a RAID 5 array with several disks loses the capacity of one disk.
Other RAID arrays are arranged so that they are faster to write to and read from than a single disk.
There are various combinations of these approaches giving different trade offs of protection against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.
RAID 0 (striped disks) distributes data across several disks in a way that gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails.
RAID 1 (mirrored disks) could be described as a backup solution, using two (possibly more) disks that each store the same data so that data is not lost as long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure nor decrease the reliability of the remaining drives (second, third, etc).
RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk.
RAID 6 (less common) can recover from the loss of two disks.
RAID 7 (experimental) single disk array in development by the Michaelis Project.
RAID 10 (or 1+0) uses both striping and mirroring.
RAID involves significant computation when reading and writing information. With true RAID hardware the controller does all of this computation work. In other cases the operating system or simpler and less expensive controllers require the host computer's processor to do the computing, which reduces the computer's performance on processor-intensive tasks (see "Software RAID" and "Fake RAID" below). Simpler RAID controllers may provide only levels 0 and 1, which require less processing.
RAID systems with redundancy continue working without interruption when one, or sometimes more, disks of the array fail, although they are vulnerable to further failures. When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally. Some systems have to be shut down when removing or adding a drive; others support hot swapping, allowing drives to be replaced without powering down. RAID with hot-swap drives is often used in high availability systems, where it is important that the system keeps running as much of the time as possible.
RAID is not a good alternative to backing up data. Data may become damaged or destroyed without harm to the drive(s) on which it is stored. For example, part of the data may be overwritten by a system malfunction; a file may be damaged or deleted by user error or malice and not noticed for days or weeks; and of course the entire array is at risk of catastrophes such as theft, flood, and fire.
Principles
RAID combines two or more physical hard disks into a single logical unit by using either special hardware or software. Hardware solutions often are designed to present themselves to the attached system as a single hard drive, and the operating system is unaware of the technical workings. Software solutions are typically implemented in the operating system, and again would present the RAID drive as a single drive to applications.
There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID levels use one or more of these techniques, depending on the system requirements. The main aims of using RAID are to improve reliability, important for protecting information that is critical to a business, for example a database of customer orders; or to improve speed, for example a system that delivers video on demand TV programs to many viewers.
The configuration affects reliability and performance in different ways. The problem with using more disks is that it is more likely that one will go wrong, but by using error checking the total system can be made more reliable by being able to survive and repair the failure. Basic mirroring can speed up reading data as a system can read different data from both the disks, but it may be slow for writing if the configuration requires that both disks must confirm that the data is correctly written. Striping is often used for performance, where it allows sequences of data to be read from multiple disks at the same time. Error checking typically will slow the system down as data needs to be read from several places and compared. The design of RAID systems is therefore a compromise and understanding the requirements of a system is important. Modern disk arrays typically provide the facility to select the appropriate RAID configuration. PC Format Magazine claims that "in all our real-world tests, the difference between the single drive performance and the dual-drive RAID 0 striped setup was virtually non-existent. And in fact, the single drive was ever-so-slightly faster than the other setups, including the RAID 5 system that we'd hoped would offer the perfect combination of performance and data redundancy"[3].Nested levels
Many storage controllers allow RAID levels to be nested: the elements of a RAID may be either individual disks or RAIDs themselves. Nesting more than two deep is unusual.
As there is no basic RAID level numbered larger than 9, nested RAIDs are usually unambiguously described by concatenating the numbers indicating the RAID levels, sometimes with a "+" in between. For example, RAID 10 (or RAID 1+0) consists of several level 1 arrays of physical drives, each of which is one of the "drives" of a level 0 array striped over the level 1 arrays. It is not called RAID 01, to avoid confusion with RAID 1, or indeed, RAID 01. When the top array is a RAID 0 (such as in RAID 10 and RAID 50) most vendors omit the "+", though RAID 5+0 is clearer.
RAID 0+1: striped sets in a mirrored set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
RAID 1+0: mirrored sets in a striped set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. In a failed disk situation, RAID 1+0 performs better because all the remaining disks continue to be used. The array can sustain multiple drive losses so long as no mirror loses all its drives.
RAID 5+0: stripe across distributed parity RAID systems.
RAID 5+1: mirror striped set with distributed parity (some manufacturers label this as RAID 53).
Non-standard levels
Many configurations other than the basic numbered RAID levels are possible, and many companies, organizations, and groups have created their own non-standard configurations, in many cases designed to meet the specialised needs of a small niche group. Most of these non-standard RAID levels are proprietary.
Some of the more prominent modifications are:
Storage Computer Corporation uses RAID 7, which adds caching to RAID 3 and RAID 4 to improve I/O performance.
EMC Corporation offered RAID S as an alternative to RAID 5 on their Symmetrix systems (which is no longer supported on the latest releases of Enginuity, the Symmetrix's operating system).
The ZFS filesystem, available in Solaris, OpenSolaris, FreeBSD and Mac OS X, offers RAID-Z, which solves RAID 5's write hole problem.
NetApp's Data ONTAP uses RAID-DP (also referred to as "double", "dual" or "diagonal" parity), which is a form of RAID 6, but unlike many RAID 6 implementations, does not use distributed parity as in RAID 5. Instead, two unique parity disks with separate parity calculations are used. This is a modification of RAID 4 with an extra parity disk.
Accusys Triple Parity (RAID TP) implements three independent parities by extending RAID 6 algorithms on its FC-SATA and SCSI-SATA RAID controllers to tolerate three-disk failure.
Linux MD RAID10 (RAID10) implements a general RAID driver that defaults to a standard RAID 1+0 with 4 drives, but can have any number of drives, including an odd number. MD RAID10 can run striped and mirrored with only 2 drives with the f2 layout (mirroring with striped reads, normal Linux software RAID 1 does not stripe reads, but can read in parallel) [5].
Infrant (Now part of Netgear) X-RAID offers dynamic expansion of a RAID5 volume without having to backup/restore the existing content. Just add larger drives one at a time, let it resync, then add the next drive until all drives are installed. The resulting volume capacity is increased without user downtime.
BeyondRAID created by Data Robotics and used in the Drobo series of products, implements both mirroring and striping simultaneously or individually dependent on disk and data context. BeyondRAID is more automated and easier to use than many standard RAID levels. It also offers instant expandability without reconfiguration, the ability to mix and match drive sizes and the ability to reorder disks. It is a block-level system and thus file system agnostic although today support is limited to NTFS, HFS+, FAT32, and EXT3. It also utilizes thin provisioning to allow for single volumes up to 16TB depending on the host operating system support.
NOTE:
没有评论:
发表评论