EMC CX500I Configuration Guide - Page 37

RAID Types and Trade-offs larger. By using both the read and write cache, a RAID 3 group can handle several concurrent streams of access. RAID 3 Groups do not require any special buffer area. No fixed memory is required to use write cache with RAID 3. Simply allocate write cache as you would for RAID 5, and ensure that caching is turned on for the LUNs in the RAID 3 Groups. Access to RAID 3 LUNs is compatible with concurrent access to LUNS of other RAID types on the storage system. RAID 1 Mirrored Pair A RAID 1 Group consists of two disks that are mirrored automatically by the storage-system hardware. With a RAID 1 Group, you can create multiple RAID 1 LUNs to apportion disk space to different users, servers, and applications. RAID 1 hardware mirroring within the storage system is not the same as software mirroring, remote mirroring, or hardware mirroring for other kinds of disks. Functionally, the difference is that you cannot manually stop mirroring on a RAID 1 mirrored pair, and then access one of the images independently. If you want to use one of the disks in such a mirror separately, you must unbind the mirror (losing all data on it), rebind the disk as the type you want, and software format the newly bound LUN. With a storage system, RAID 1 hardware mirroring has the following advantages: ◆ automatic operation (you do not have to issue commands to initiate it) ◆ physical duplication of images ◆ a rebuild period that you can select during which the SP recreates the second image after a failure RAID Types 2-7