Dell 8 Manuale d'uso - Pagina 34

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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 physical disks are accessed

simultaneously, but disk striping does not provide data redundancy.

Figure 3. Example of Disk Striping (RAID 0)

Disk Mirroring

With mirroring (used in RAID 1), data written to one disk is simultaneously written to another disk. If one disk 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 contain the 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 in the system must

be duplicated.

NOTE: Mirrored physical disks improve read performance by read load balance.

Figure 4. Example of Disk Mirroring (RAID 1)

Spanning is a term used to describe the way in which RAID levels 10, 50, and 60 are constructed from multiple sets of

basic, or simple RAID levels. For example, a RAID 10 has multiple sets of RAID 1 arrays where each RAID 1 set is

considered a span. Data is then striped (RAID 0) across the RAID 1 spans to create a RAID 10 virtual disk. Similarly, RAID

50 and RAID 60 combine multiple sets of RAID 5 or RAID 6 respectively with striping.

Parity data is redundant data that is generated to provide fault tolerance within certain RAID levels. In the event of a disk

failure, the parity data can be used by the controller to regenerate user data. Parity data is present for RAID 5, 6, 50, and

60.