system, the operating system implements the RAID levels. For this reason, using software RAID by itself can slow the systemperformance. You can, however, use software RAID along with hardware RAID volumes to provide better performance and varietyin the configuration of RAID volumes. For example, you can mirror a pair of hardware RAID 5 volumes across two RAID controllers toprovide RAID controller redundancy.RAID conceptsRAID uses particular techniques for writing data to disks. These techniques enable RAID to provide data redundancy or betterperformance. These techniques include:• Mirroring — Duplicating data from one physical disk to another physical disk. Mirroring provides data redundancy by maintainingtwo copies of the same data on different physical disks. If one of the disks in the mirror fails, the system can continue to operateusing the unaffected disk. Both sides of the mirror contain the same data always. Either side of the mirror can act as theoperational side. A mirrored RAID disk group is comparable in performance to a RAID 5 disk group in read operations but faster inwrite operations.• Striping — Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual disk dataaddresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern. For example, ifthe virtual disk includes five physical disks, the stripe writes data to physical disks one through five without repeating any of thephysical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion of a stripe that resideson a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in combination with paritydoes provide data redundancy.• Stripe size — The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that contains64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe size is 64KB and the stripeelement size is 16KB.• Stripe element — A stripe element is the portion of a stripe that resides on a single physical disk.• Stripe element size — The amount of disk space consumed by a stripe element. For example, consider a stripe that contains64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe element size is 16KB and thestripe size is 64KB.• Parity — Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of thestriped disks fails, the data can be reconstructed from the parity information using the algorithm.• Span — A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10, 50, or 60virtual disk.RAID levelsEach RAID level uses some combination of mirroring, striping, and parity to provide data redundancy or improved read and writeperformance. For specific information on each RAID level, see Choosing raid levels.Organizing data storage for availability and performanceRAID provides different methods or RAID levels for organizing the disk storage. Some RAID levels maintain redundant data so thatyou can restore data after a disk failure. Different RAID levels also entail an increase or decrease in the I/O (read and write)performance of a system.Maintaining redundant data requires the use of additional physical disks. The possibility of a disk failure increases with an increase inthe number of disks. Since the differences in I/O performance and redundancy, one RAID level may be more appropriate thananother based on the applications in the operating environment and the nature of the data being stored.When choosing a RAID level, the following performance and cost considerations apply:• Availability or fault-tolerance — Availability or fault-tolerance refers to the ability of a system to maintain operations and provideaccess to data even when one of its components has failed. In RAID volumes, availability or fault-tolerance is achieved bymaintaining redundant data. Redundant data includes mirrors (duplicate data) and parity information (reconstructing data usingan algorithm).• Performance — Read and write performance can be increased or decreased depending on the RAID level you choose. SomeRAID levels may be more appropriate for particular applications.• Cost efficiency — Maintaining the redundant data or parity information associated with RAID volumes requires additional diskspace. In situations where the data is temporary, easily reproduced, or non-essential, the increased cost of data redundancy maynot be justified.184
