RAID 1

RAID 1

In level 1, drives are paired or mirrored with each byte of information being written to each identical drive. You can duplex these devices by adding a separate drive controller for each drive (duplexing is examined later in this chapter). Disk mirroring is defined as two hard drives—one primary, one secondary—that use the same disk channel (controller cards and cable), as illustrated in Figure 9.4. Disk mirroring is most commonly configured by using disk drives contained in the server. Duplexing, which is covered later in this chapter, is a form of mirroring that enables you to configure a more robust hardware environment.

Mirroring does not provide a performance benefit such as RAID 0 provides. Use mirroring, however, to create two copies of the server’s data and operating system, which enables either disk to boot and run the server. If one drive in the pair fails, for instance, the other drive can continue to operate. Disk mirroring can be expensive, though, because it requires 2 GB of disk space for every 1 GB you want to mirror. You also must make sure that your power source has enough wattage to handle the additional devices. Mirroring requires two drives, and the mirrored partitions must be of the same size. Windows NT Server supports mirroring, but Windows NT Workstation and Windows 95 do not.

Remember that mirroring is done for fault-tolerant, not performance reasons. With this said, it should be noted that an NT machine running a mirror set will run at about normal speed on writes to the mirror set, but can have marginal performance gains reading from the set. For the best of both worlds, though, we need to move on to RAID 5.

RAID 2, 3, 4, and 5 are all versions of striping that incorporate similar fault-tolerant designs. Microsoft chose to support only level 5 striping in Windows NT Server. As the numbering scheme would imply, this is the newest revision of the four and is the most popular fault-tolerance scheme in use today. Level 5 requires less disk space than mirroring and has performance gains over other striping methods. As with mirroring, RAID level 5 is not available in Windows NT Workstation or Windows 95.