EMC CX500I Configuration Guide - Page 41

Hot Spare RAID Types and Trade-offs A hot spare is a dedicated replacement disk on which users cannot store information. A hot spare is global: if any disk in a RAID 5 Group, RAID 3 Group, RAID 1 mirrored pair, or RAID 1/0 Group fails, the SP automatically rebuilds the failed disk's structure on the hot spare. When the SP finishes rebuilding, the disk group functions as usual, using the hot spare instead of the failed disk. When you replace the failed disk, the SP copies the data from the former hot spare onto the replacement disk. When the copy is done, the disk group consists of disks in the original slots, and the SP automatically frees the hot spare to serve as a hot spare again. A hot spare is most useful when you need the highest data availability. It eliminates the time and effort needed for someone to notice that a disk has failed, find a suitable replacement disk, and insert the disk. When you plan to use a hot spare, make sure the disk has the capacity to serve in any RAID Group in the storage-system chassis. A RAID Group cannot use a hot spare that is smaller than a failed disk in the group. You can have one or more hot spares per storage-system chassis. You can make any disk in the chassis a hot spare, except a disk that stores the FLARE™ operating environment (formerly called Base or Core Software) or the write cache vault; that is, a hot spare can be any disk except disk IDs 000-004. If you use hot spares of different sizes, the storage system will automatically use the hot spare of the proper size in place of a failed disk. An example of hot spare usage for a CX700 storage system follows. RAID Types 2-11