RAID is a method of combining several hard drives into one unit. It can offer fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives. The following information explains the different RAID levels.
RAID 0 provides improved performance, but no fault tolerance. Because the data is striped across more than one disk, RAID 0 disk arrays achieve high transfer rates because they can read and write data on more than one drive simultaneously. You can configure the stripe size during unit creation. RAID 0 requires a minimum of two drives.
Striped disk arrays give exceptional performance, particularly for data-intensive applications such as video editing, computer-aided design, and geographical information systems.
RAID 0 arrays are not fault tolerant. The loss of any drive results in the loss of all the data in that array, and can even cause a system hang, depending on your operating system. RAID 0 arrays are not recommended for high-availability systems unless you take additional precautions to prevent system hangs and data loss.
RAID 1 provides fault tolerance and a speed advantage over non-RAID disks. RAID 1 also is known as a mirrored array. Mirroring is done on pairs of drives. Mirrored disk arrays write the same data to two different drives using RAID 1 algorithms. This gives your system fault tolerance by preserving the data on one drive if the other drive fails. Fault tolerance is a basic requirement for critical systems should as web and database servers.
3ware firmware uses a patented TwinStor technology, on RAID 1 arrays for improved performance during sequential read operations. With TwinStor technology, read performance during a sequential read operation is twice the speed of a single drive.
The adaptive algorithms in TwinStor technology boost performance by distinguishing between random read request and sequential read requests. For the sequential read requests generated when accessing large files, both drives are used with the drive heads simultaneously reading alternating sections of the file. For the smaller random transactions, the data is read by a single optimal drive head.
RAID 5 provides performance, fault tolerance, high capacity, and storage efficiency. It requires a minimum of three drives and combines striping data with parity (exclusive OR) to restore data in case of a drive failure. Performance and efficiency increase as the number of drives in a unit increases.
Parity information is distributed across all of the drives in a unit rather than being concentrated on a single disk. This method avoids throughput loss due to contention for the parity drive.
RAID 6 provides greater redundancy and fault tolerance than RAID 5. It is similar to RAID 5 but, instead of a single block, RAID 6 has two blocks of parity information (P+Q) distributed across all the drives of a unit (see
Figure: Show RAID 6 Configuration Example).
Due to the two parities, a RAID 6 unit can tolerate two hard drives failing simultaneously. This also means that a RAID 6 unit can be in two different states at the same time. For example, one subunit can be degraded while another is rebuilding, or one subunit can be initializing while another is verifying.
The 3ware implementation of RAID 6 requires a minimum of five drives. Performance and storage efficiency also increase as the number of drives increase.
When drives are configured as a striped mirrored array, the disks are configured using both RAID 0 and RAID 1 techniques. A minimum of four drives are required to use this technique. The first two drives are mirrored as a fault-tolerant array using RAID 1. The third and fourth drives are mirrored as a second fault-tolerant array using RAID 1. The two mirrored arrays are then grouped as a striped RAID 0 array using a two-tier structure. Higher data transfer rates are achieved by leveraging TwinStor technology and striping the arrays.
In addition, RAID 10 arrays offer a higher degree of fault tolerance than RAID 1 and RAID 5 because the array can sustain multiple drive failures without data loss. For example, in a 12-drive RAID 10 array, up to 6 drives can fail (half of each mirrored pair) and the array continues to function. Note that if both halves of a mirrored pair in the RAID 10 array fail, all of the data is lost.
RAID 50 is a combination of RAID 5 and RAID 0. This array type provides fault tolerance and high performance. RAID 50 requires a minimum of six drives.
Several combinations are available with RAID 50. For example, on a 12-port controller, you can have a grouping of three, four, or six drives. A grouping of three means that the RAID 5 arrays used have three disks each; four of these 3-drive RAID 5 arrays are striped together to form the 12-drive RAID 50 array. On a 16-port controller, you can have a grouping of four or eight drives.
No more than four RAID 5 subunits are allowed in a RAID 50 unit. For example, a 24-drive RAID 50 unit may have groups of 12, eight, or six drives, but not groups of four or three (see
Figure: Show RAID 50 Configuration Example).
In addition, RAID 50 arrays offer a higher degree of fault tolerance than RAID 1 and RAID 5, because the array can sustain multiple drive failures without data loss. For example, in a 12-drive RAID 50 array, one drive in each RAID 5 set can fail and the array continues to function. Note that if two or more drives in a RAID 5 set fail, all of the data is lost.
Similar to disks in other RAID configurations, single disks contain 3ware Disk Control Block (DCB) information and the OS addresses them as available units.
Single drives are not fault tolerant and, therefore, are not recommended for high availability systems unless you take additional precautions to prevent system hangs and data loss.
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