Accusys ExaSAN A08S3-PS User Manual
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RAIDUser Guide 5-2remove and replace a failed disk (known as “hot swapping”), and the controller then rebuilds thedata using the information on the remaining drives.5.3 RAID LevelsThe ExaSAN RAID system supports several RAID levels and configurations. Each level has adifferent architecture and provides varying degrees of performance and fault tolerance. Eachlevel has characteristics to achieve maximum performance or redundancy depending on thedata environment.5.3.1 RAID 0: StripingRAID level 0, striping only, is the fastest and most efficient array type, but offer no fault-tolerance. Any drive failure destroys the data in the array.5.3.2 RAID 1: MirroringRAID level 1, mirroring, has been used for Metadata LUN because of its simplicity and highlevels of reliability and availability. Mirroring uses two drives, each drive stores identical data.RAID 1 provides very high data reliability and improved performance for read-intensiveapplications, but this level has a high capacity cost because it retains a full copy of your data oneach drive in mirror set.In a RAID 1 configuration, the capacity of the smallest drive is the maximum storage area.5.3.3 RAID 5: Independent data disks with distributed parityBy distributing the parity information across all drives in a set, RAID level 5 achieves highreliability and data availability. It also offers the highest read data transaction rate of all levelsalong with a medium write rate. The low ratio of ECC (Error Correction Code) parity disks todata disks offers hardware efficiency. Disk failure has a moderate impact on the total transferrate.5.3.4 RAID 6: Independent data disks with two Independent parity schemesRAID level 6 extends RAID level 5 by adding an additional parity block; thus it uses block-levelstriping with two parity blocks distributed across all member disks. RAID 6 does not have aperformance penalty for read operations, but it does have a performance penalty on writeoperations because of the overhead associated with parity calculations.RAID 6 is no more space inefficient than RAID 5 with a hot spare drive when used with a smallnumber of drives, but as arrays become bigger and have more drives the loss in storagecapacity becomes less important and the probability of data loss is greater. RAID 6 providesprotection against data loss during an array rebuild, when a second drive is lost, a bad blockread is encountered, or when a human operator accidentally removes and replaces the wrongdisk drive when attempting to replace a failed drive.5.3.5 RAID 0+1: Striped set with MirroringRAID 0+1 combines the advantages of RAID 0 and RAID 1 with none of the disadvantages.RAID 0+1 creates a mirror of the primary striped set. RAID 0+1 provides optimal speed andreliability. |
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