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Various RAID Levels

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» For RAID 0 (Data Striping), the cost of storage is higher than for a single disk (assuming that a single disk has sufficient capacity) since using several disks (regardless of their ability to provide more storage capacity than a single disk) increases costs for items such as racks, cables, controllers, power. Data availability is lower than for a single
disk, because MTBF for the RAID is the MTBF of a single disk divided by the number of disks used—that is, a RAID 0 of N disks has an MTBF N times smaller than the MTBF of a single disk. Reading and writing large blocks on a RAID 0 using N disks takes less time than for a single disk (at best N times less, limited by the fact that the disks are not in general rotationally synchronized). This reduces the occupation time of the disks and allows higher bandwidths. The same is true for random reads and writes.


» For RAID 1 (Mirroring), the storage cost is proportional to the number of copies of the data kept (the factor M in the table). Most often, mirroring is simple replication (M = 2). As to availability, it is clear that RAID 1 has higher availability than RAID 3 or RAID 5, since it has complete data replication rather than a parity disk per N physical disks. Reading, whether of large transfers or random transfers, has higher performance because the data can be read concurrently from multiple disks. Concurrency is less effective for writes, whether for large transfers or random transfers, because of the need to not signal completion until the last write on the last disk is complete.


» RAID 0 + 1 (Striped Mirror) has more or less the same properties as RAID 1, with just one further comment on write operations: the time for write operations for large transfers can be lower than for a single disk, if the time saved as a result of distributing the data across N parallel disks is greater than the extra cost of synchronizing completion across M groups of disks.


» RAID 3 (Parity Disk) availability is ensured through the use of parity information. Large block reads offer similar performance to RAID 0, with any differences attributable to the need to compute parity for the information read, along with any required correction. Large block writes are slower, because such transfers involve both the calculation of parity and writing the parity values to the parity disk, whose busy time can be greater than those of the collection of data disks, since there is just one parity disk. Random reads require a parity disk access, calculation of data parity, parity comparison, and any necessary correction. A write operation implies calculation of parity and its writing to the parity disk. Performance compared with a single disk depends on the performance advantage obtained by distributing the data across multiple disks.


» RAID 5 (Spiral Parity) offers essentially the same availability as RAID 3. Again, large transfer performance is impacted by the need to calculate parity and apply correction as required. Random reads and writes are generally better than for RAID 3 because of the distribution of parity information over multiple disks, reducing contention on parity updates.


» RAID 6 (Double Spiral Parity) provides higher availability than RAID 5, since it can survive two concurrent independent failures. RAID 6 has slightly higher read performance than RAID 5, since double parity reduces contention and thus wait time for parity writes (only slightly higher performance, since the number of disks grows only from N + 1 to N + 2). Write operations, on the other hand, are slower, suffering from the increased burden of double parity computation and writing.

Source of Information : Elsevier Server Architectures

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