RAID (Redundant Array of Independents Disks) is a simple technology that improves the performance of external storage solutions. RAID allows you to choose the best way to use your device to fit your needs. Simply put, RAID technology either divides or duplicates the task of one hard disk between many (or as few as two) disks to either improve performance or create data redundancy in case of a drive failure. You can decide how the device handles data by setting its RAID mode.
To better understand how RAID works, first familiarize yourself with the terms below:
✦Striping is the splitting of data between multiple drives. Striped RAID arrays generally aim to merge maximum capacity into one single volume.
✦Mirroring is the copying of data to more than one disk. Mirrored RAID arrays typically allow the failure of at least one disk in the array without any data loss, depending on the RAID level of the array.
✦Fault Tolerance allows a RAID array to continue working (i.e. the data stored in the array is still available to the user) in the event of a disk failure. Not all mirrored RAID arrays are user friendly. For instance, some RAID devices must be shut down before a failed disk is replaced, whereas LaCie RAID devices feature “hot-swappaple” disk replacement that allows the device to remain on, and the data accessible, while the failed disk is replaced.
RAID 0
RAID 0 (also called FAST mode in some LaCie devices), is the fastest RAID mode. Requiring at least 2 drives, RAID 0 stripes data onto each disk. The available capacities of each disk are added together so that one logical volume mounts on the computer.
If one physical disk in the array fails, the data of all disks becomes inaccessible because parts of the data have been written to all disks.
RAID 1RAID 1 (also called SAFE mode in some LaCie devices) is a secure RAID mode that requires at least 2 drives and works with pairs of drives. One logical volume mounts on the computer and the combined available capacity of both drives is limited to the capacity of the lowest-capacity disk. If one physical disk fails, the data is available immediately on the second disk. No data is lost if one disk fails.
RAID 2 No practical use.
Minimum number of drives: Not used in LAN
Strengths: Previously used for RAM error environments correction (known as Hamming Code ) and in disk drives before he use of embedded error correction.
Weaknesses: No practical use; Same performance can be achieved by RAID 3 at lower cost.
RAID 3RAID 3 uses byte-level striping with a dedicated parity disk (Disk 4 in the illustrations, right) so that one volume mounts on the computer. A RAID 3 array can tolerate a single disk failure without data loss. If one physical disk fails, the data from the failed disk can be rebuilt onto a replacement disk. If a second disk fails before data can be rebuilt to a replacement drive, all data in the array will be lost.
RAID 3+SpareIn RAID 3+Spare, one disk in the array is left empty. If a drive in the array fails, the data from the failed disk is automatically rebuilt to the empty or “spare” disk.
RAID 5RAID 5 combines the striping of RAID 0 with data redundancy in an array with a minimum of three disks.
The difference between RAID 3 and RAID 5 is that a RAID 3 configuration will offer better performance at the expense of slightly less overall capacity. Data is striped across all disks and a parity block (P) for each data block is written on the same stripe. If one physical disk fails, the data from the failed disk can be rebuilt onto a replacement disk. No data is lost in the case of a single disk failure, but if a second disk fails before data can be rebuilt to a replacement drive, all data in the array will be lost.
