3.2 Choose a disaster recovery plan for various situations.
RAID means Redundant Arrays of Inexpensive Disks. Windows NT server supports RAID 0, 1 and 5. Windows NT workstation does not support RAID.
RAID 0 is just disk striping. It includes a disk array that implements striping without any drive redundancy. It offers no fault tolerance and is less reliable than a single-drive implementation; its only advantage is speed. RAID 0 is suitable for certain special applications, as in scientific analysis or imaging, where compromised system reliability can be tolerated.
Disk Mirroring (RAID Level 1) uses two partitions on different drives connected to the same disk controller. All data on the first (primary) partition is mirrored automatically onto the secondary partition. Thus, if the primary disk fails, no data is lost. Instead, the partition on the secondary disk is used.
For every disk operation, the system must write the same information to both disks. Because dual write operations can degrade system performance, many systems utilize duplexing, where each mirror drive has it own host adapter. While the mirror approach provides good fault tolerance, it is relatively expensive to implement, because only half of the available disk space can be used for storage while the other is used for mirroring.
Mirroring is not restricted to a partition identical to the primary partition in size, number of tracks and cylinders, and so on. This eliminates the problem of acquiring an identical model drive to replace a failed drive when an entire drive is being mirrored. For practical purposes though, the mirrored partitions will usually be created to be the same size as the primary partition.
Disk mirroring has better overall read and write performance than stripe sets with parity. Another advantage of mirroring over stripe sets with parity is that there is no loss in performance when a member of a mirror set fails. Disk mirroring, however, is more expensive in terms of dollars per megabyte because disk utilization is lower than with striping with parity. Disk mirroring is best suited for peer-to-peer and modest server-based LANs.
Disk Duplexing is simply a mirrored pair with an additional controller on the secondary drive. Duplexing provides fault tolerance for both disk and controller failure. In addition to providing fault tolerance, it can also improve performance.
Like mirroring, duplexing is performed at the partition level. To the Windows NT Server operating system, there is no difference between mirroring and duplexing. It is simply a matter of where the other partition can be found.
Disk Striping with Parity (RAID Level 5) is another popular method of protecting data against disk failure. With disk striping, data is divided into large blocks and spread in a fixed order among multiple disks in an array. In a stripe set with parity, parity information for the data is also written across the array with the condition that the parity information and data reside on different disks. If a member of the disk array fails, data can be recovered from the parity information since it is stored on a different disk. One advantage of stripe sets with parity is that they have better read performance (although slower write performance) than mirror sets. Another advantage is that the cost per stored megabyte is typically lower with stripe sets with parity than with mirrored sets because disk utilization is much higher.
To fully understand Windows NT Server-based disk striping, it is valuable to compare it to hardware-based striping systems. Hardware implementation of the RAID level can offer performance advantages over software implementations. With some systems, it may even be possible to replace a failed drive without shutting down the system. However, hardware RAID implementations tend to be very expensive and may require an organization to lock-in to a single vendor solution. The RAID technology of Windows NT Server is powerful and cost-effective and provides a consistent implementation across numerous hardware platforms. Windows NT Servers RAID technology provides greater flexibility in mixing systems that provide optimum price and performance for customer needs.
RAID 5 dedicates the equivalent of one entire disk for storing check data but distributes the check data over all the drives in the group. For example, sector 1 of disk 5 may be assigned to hold the check data for sector 1 of the remaining data drives and so on.
The check data is simply the XOR of all the write data values for the corresponding sector on each of the data disks. As long as the old sector data and the old check data values are known, the new check data for a single sector write can be calculated without having to read the corresponding sectors from the other data disks. Thus, only two disks are involved in a single sector write operation: the target data disk and the corresponding disk that holds the check data for that sector.
The primary benefit of the RAID 5 distributed check data approach is that it permits write operations to take place simultaneously. It also allows multiple reads to take place simultaneously and is efficient in handling small amounts of information.
Sector sparing involves the automatic maintenance of a spare sector per track to be used if another sector becomes defective. Windows NT supports sector sparing, when a bad sector is found during disk I/O, it will attempt to store the data to a good sector and map the bad sector.
Sector sparing functions somewhat like Scandisk. Scandisk can only mark the bad sector. It cannot recover the data as sector sparing does. Sector sparing also needs hardware support. It can be only implemented with SCSI devices and not with AT devices (such as IDE and ESID devices). Thats the one reason why most high performance networking servers use SCSI hard disk while not the IDE disk.