Averting disaster with redundant hardware.

• Author: Maida, Joseph C.
• Position: Computer hardware

Hard disk failures can be catastrophic; they can necessitate three days of downtime while a replacement drive arrives and is installed, and a backup tape restores the data. It can take as much as three weeks to recover data initially, arid three months to recover it completely.

Initial recovery replaces damaged data or programs on a priority basis. In the remaining months, data or programs of minor value can be replaced.

While backups are traditionally the mainstay of recapturing and restoring lost data, they are not foolproof for restoring failed drives; the last day's data is generally missing and the damaged portions of the drive that are faithfully being backed up are of no value when restored.

RAID Subsystems

The crisis that results from a hard drive failure can be averted by the purchase of a redundant array of inexpensive disks (RAID) subsystem, which can remove a single failure point by spreading the risk of hard disk failure over multiple disks instead of one. The redundancy offered by multiple disks (offsets the lower reliability of a single disk. RAID can be either hardware-based or software-simulated. A hardware-based RAID system provides more robust features than its slower software-based simulated alternative. RAID offers various mixes of performance, reliability and cost, which have been standardized and categorized in six levels known as RAID levels 0 through 5. (The Microsoft Technet document entitled Reliability and Fault Tolerance, which describes RAID as implemented ill the Microsoft Windows NT Server Network Operating System, can be found at http:// technet.microsofk.com/cdonline/ content/complete/boes/bo/winntas/ technote/reliabil.htm.)

RAID fault tolerance. RAID allows restoration of information on an old drive without a backup tape. In fact, when a failing drive is removed, a server continues to operate normally and no data is lost due to the drive's removal. All this is possible because the RAID hard disk controller reconstructs the missing information from the information that resides on the remaining redundant healthy disks.

Disk striping. A RAID controller keeps track of the different physical disks and allocates storage space on all of these disks in a particular manner called striping. Striping allows the contents of a single data file or program to be spread over three or more disks. This can provide faster access to the data than if the file were solely located within one drive. If one drive fails, the...