For example, in a four-disk system using only disk striping (used in RAID 0), segment 1 is written to disk 1,segment 2 is written to disk 2, and so on. Disk striping enhances performance because multiple physicaldisks are accessed simultaneously, but disk striping does not provide data redundancy.Figure 6. Example of Disk Striping (RAID 0)Disk MirroringWith mirroring (used in RAID 1), data written to one disk is simultaneously written to another disk. If onedisk fails, the contents of the other disk can be used to run the system and rebuild the failed physical disk.The primary advantage of disk mirroring is that it provides complete data redundancy. Both disks containthe same data at all times. Either of the physical disks can act as the operational physical disk.Disk mirroring provides complete redundancy, but is an expensive option because each physical disk inthe system must be duplicated.NOTE: Mirrored physical disks improve read performance by read load balance.Figure 7. Example of Disk Mirroring (RAID 1)Spanned RAID LevelsSpanning is a term used to describe the way in which RAID levels 10, 50, and 60 are constructed frommultiple sets of basic, or simple RAID levels. For example, a RAID 10 has multiple sets of RAID 1 arrayswhere each RAID 1 set is considered a span. Data is then striped (RAID 0) across the RAID 1 spans tocreate a RAID 10 virtual disk. Similarly, RAID 50 and RAID 60 combine multiple sets of RAID 5 or RAID 6respectively with striping.Parity DataParity data is redundant data that is generated to provide fault tolerance within certain RAID levels. In theevent of a disk failure, the parity data can be used by the controller to regenerate user data. Parity data ispresent for RAID 5, 6, 50, and 60.98
