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What is Redundant Arrays of Inexpensive Disks (RAID)
In 1987, the University of California Berkeley, published an article entitled A Case for Redundant Arrays of Inexpensive Disks (RAID). This article described various types of disk arrays, referred to by the acronym RAID. The basic idea of RAID was to combine multiple small, independent disk drives into an array of disk drives which yields performance.
Additionally, this array of drives appears to the computer as a single logical storage unit or drive.
The RAID levels
The RAID Advisory Board defined seven standard RAID levels, called RAID 0–6. Most RAID controllers also implement a RAID 0+1 combination, which is usually called RAID 10. The levels are as follows:
RAID Level 0— Also called "stripe" mode. Striping-File data is written simultaneously to multiple drives in the array, which act as a single larger drive. This offers high read/write performance but low reliability. Operations on the array will be split on the devices; for example, a large write could be split up as 4 kB to disk 0, 4 kB to disk 1, 4 kB to disk 2, then 4 kB to disk 0 again, and so on. Requires a minimum of two drives to implement. The devices should (but need not) have the same size. If one device is much larger than the other devices, that extra space is still utilized in the RAID device, but you will be accessing this larger disk alone, during writes in the high end of your RAID device. This of course hurts performance. The read and write performance will increase, because reads and writes are done in parallel on the devices. This is usually the main reason for running RAID-0.
RAID Level 1—Mirroring-Data written to one drive is duplicated on another, providing excellent fault tolerance (if one drive fails, the other is used and without the data lost) but no real increase in performance as compared to a single drive. Requires a minimum of two drives to implement (same capacity as one drive). Of Course, the disks must be of equal size. If one disk is larger than another, your RAID device will be the size of the smallest disk.
RAID Level 2—Bit-level ECC-Data is split one bit at a time across multiple drives, and error correction codes (ECCs) are written to other drives. This is intended for storage devices that do not incorporate ECC internally. (All SCSI and ATA drives have internal ECC.) It’s a standard that theoretically provides high data rates with good fault tolerance, but seven or more drives are required for greater than 50% efficiency, and no commercial RAID 2 controllers or drives without ECC are available.
RAID Level 3—Striped with parity-Combines RAID Level 0 striping with an additional drive used for parity information. This RAID level is really an adaptation of RAID Level 0 that sacrifices some capacity, for the same number of drives. However, it also achieves a high level of data integrity or fault tolerance because data usually can be rebuilt if one drive fails. Requires a minimum of three drives to implement (two or more for data and one for parity).
RAID Level 4—Blocked data with parity. It is similar to RAID 3 except data is written in larger blocks to the independent drives, offering faster read performance with larger files. Instead of
completely mirroring the information, it keeps parity information on one drive, and writes data to the other disks in a RAID-0 like way. Requires a minimum of three drives to implement (two or more for data and one for parity). If one drive fails, the parity information can be used to reconstruct all data. If two drives fail, all data is lost. This RAID level is not used very often. The reason this level is not more frequently used, is because the parity information is kept on one drive. This information must be updated every time one of the other disks are written to. Thus, the parity disk will become a bottleneck, if it is not a lot faster than the other disks.
RAID Level 5—Blocked data with distributed parity. It is similar to RAID 4 but offers improved performance by distributing the parity stripes over a series of hard drives. Requires a minimum of three drives to implement (two or more for data and one for parity). This is perhaps the most useful RAID mode when one wishes to combine a larger number of physical disks, and still maintain some redundancy. If one of the disks fail, all data are still intact, thanks to the parity information. If spare disks are available, reconstruction will begin immediately after the device failure. If two disks fail simultaneously, all data are lost. RAID-5 can survive one disk failure, but not two or more. Both read and write performance usually increase, but can be hard to predict how much.
RAID Level 6—Blocked data with double distributed parity. It is similar to RAID 5 except parity information is written twice using two parity schemes to provide even better fault tolerance in case of multiple drive failures. Requires a minimum of four drives to implement (two or more for data and two for parity).
RAID 7 is a type of RAID level that includes a real-time embedded operating system and processor for enhanced data read/write or I/O operations and data caching capabilities. It is a propriety RAID level owned by Storage Computer Corporation. RAID 7 primarily incorporates features from RAID level 3 and 4. RAID 7 has integrated cache and a purpose-built processor for managing the array that helps in achieving faster data read/write operations. It also has lesser dependency on parity disks due to the addition of controller hardware (cache and processor). Being a propriety technology, it requires a specialized controller to read/write data. RAID 7 provides triple parity.
There are also nested RAID levels created by combining several forms of RAID. The most common are as follows:
RAID Level 01: Mirrored stripes—Drives are first combined in striped RAID 0 sets; then the RAID 0 sets are mirrored in a RAID 1 configuration. A minimum of four drives is required, and the total number of drives must be an even number. Most PC implementations allow four drives only. The total usable storage capacity is equal to half of the number of drives in the array times the size of the lowest capacity drive. RAID 01 arrays can tolerate a single drive failure and some (but not all) combinations of multiple drive failures. This is not generally recommended because RAID 10 offers more redundancy and performance.
RAID Level 10: Striped mirrors—Drives are first combined in mirrored RAID 1 sets; then the RAID 1 sets are striped in a RAID 0 configuration. A minimum of four drives is required, and the total number of drives must be an even number. Most PC implementations allow four drives only. The total usable storage capacity is equal to half of the number of drives in the array times the size of the lowest capacity drive. RAID 10 arrays can tolerate a single drive failure and many (but notall) combinations of multiple drive failures. This is similar to RAID 01, except with somewhat increased reliability because more combinations of multiple drive failures can be tolerated, and rebuilding an array after a failed drive is replaced is much faster and more efficient.
When set up for maximum performance, arrays typically run RAID Level 0, which incorporates data striping. Unfortunately, RAID 0 also sacrifices reliability such that if any one drive fails, all data in the array is lost. The advantage is in extreme performance. With RAID 0, performance generally scales up with the number of drives you add in the array. For instance, with four drives you won't really have four times the execution of a solitary drive, yet numerous controllers can approach that for sustained transfers. Some overhead is as yet associated with the controller playing out the striping, issues still exist with latency—that is, how long it takes to find the data—but performance will be higher than any single drive can normally achieve.
When set up for reliability, arrays generally run RAID Level 1, which is simple drive mirroring. All information kept in touch with one drive is composed to the next. In the event that one drive fails, the framework can keep on working on the other drive. This does not increase performance, and it also means you get to use only half of the available drive capacity. In other words, you must install two drives, but you get to use only one. (The other is the mirror.) However, in a time of high limits and low drive costs, this isn't a huge issue.
Combining performance with fault tolerance requires using one of the other RAID levels, such as RAID 5 or 10.
With four 500GB drives in a RAID 5 configuration, you would have 1.5TB of total storage, and you could withstand the failure of any single drive. After a drive failure, data could still be read from and written to the array. However, read/write performance would be exceptionally slow, and it would remain so until the drive was replaced and the array was rebuilt. The rebuild process could take a relatively long time, so if another drive failed before the rebuild completed, all data would be lost.
With four drives in a RAID 10 configuration, you would have only 1TB of total storage. However, you could withstand many cases of multiple drive failures. In addition, after the failed drive is replaced, the rebuild process would go relatively quickly as compared to rebuilding a RAID 5 array. Because of the advantages of RAID 10, many are recommending it as an alternative to RAID 5 where maximum redundancy and performance are required.
Some operating systems include software-based RAID capability; in fact, limited RAID 0, 1, and even RAID 5 functionality has been built in to some versions of Windows since Windows 2000. Microsoft released Windows Home Server in 2007 it significantly upgraded this ability with a component called Drive Extender, which took into consideration the creation and self-assertive extension of an exhibit utilizing for all intents and purposes any kind of drive (SATA, PATA, USB, FireWire, et cetera) in any way.
This is similar to a RAID setup but drive extender had a few cards up it’s sleeve that made it invaluable for a NAS appliance.
- No special RAID controller or hardware was needed.
- Hard drives did not have to match in size or manufacturer so you could literally take any hard drive and use it in your pool.
- Hard drives used standard NTFS filesystems. If something bad happened, you could just pull out the drive and plug it into another computer to view and recover files.
Few problems with Drive Extender caused Microsoft to remove the feature from Windows Home Server 2011.
Microsoft has incorporated a more current and better trade for Drive Extender in Windows 8/8.1/10, which is presently called Storage Spaces. Just like Drive Extender, it enables you to build a virtual drive using an array of drives of just about any type or capacity. One area where Storage Spaces differs from Drive Extender is in the redundancy options. In addition to two-way redundancy where data is saved on two drives, Storage Spaces allows for three-way redundancy, meaning that data will be saved on three drives. This also means that up to two drives can fail in the array without losing data.
Software-based RAID, performance falls dramatically as compared to either a physical drive or hardware-based RAID, especially in write performance.
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