Brought to you by EarthWeb
IT Library Logo

Click Here!
Click Here!


Search the site:
 
EXPERT SEARCH -----
Programming Languages
Databases
Security
Web Services
Network Services
Middleware
Components
Operating Systems
User Interfaces
Groupware & Collaboration
Content Management
Productivity Applications
Hardware
Fun & Games

EarthWeb Direct EarthWeb Direct Fatbrain Auctions Support Source Answers

EarthWeb sites
Crossnodes
Datamation
Developer.com
DICE
EarthWeb.com
EarthWeb Direct
ERP Hub
Gamelan
GoCertify.com
HTMLGoodies
Intranet Journal
IT Knowledge
IT Library
JavaGoodies
JARS
JavaScripts.com
open source IT
RoadCoders
Y2K Info

Previous Table of Contents Next


Mirroring

Modern operating systems that are built for the enterprise, such as Windows NT 4.0, provide both RAID 1 and RAID 5 fault-tolerance protection. RAID level 1 (disk mirroring) simultaneously streams data to two hard drives (or tape devices) throughout the entire job, not just in the event of the hardware failure. Both drives are considered to be one drive by the Windows NT software (NT’s fault tolerance driver is called FTDISK.SYS).

Disk mirroring creates a duplication of partition data onto another physical disk. Any partition, including the boot or system partitions, can be mirrored. This strategy protects a single disk against failure.

Disk mirroring is generally appropriate for small LANs, as its initial cost is limited to only two disk drives. As the need for more network storage capacity grows, mirroring may become more expensive per megabyte than other forms of fault tolerance because only 50% of the disk space is being used. However, with the advent of low-cost integrated drive electronics (IDE) (1GB for under $300, for example), disk-mirroring controllers and large mirrored RAID drives could become viable alternatives, in workstations and small work groups, to the faster, more powerful small computer system interface (SCSI)-based arrays—provided users do not need the full performance of the SCSI systems.

For extra security, an entire RAID system can also be mirrored, but of course that doubles the cost. How communications managers determine whether mirroring or striping is best for their enterprise depends on their confidence in the technology and how much they are willing to spend.

Mirror drive sets provide a modest performance increase for reading because the fault tolerance driver can read from both members of the mirror set at the same time. However, there is a slight performance decrease when writing to a mirror set because the fault tolerance driver must write to both members simultaneously. When one drive in a mirror set fails, performance returns to normal because the fault tolerance driver is working with only one partition.

If a mirror set’s disk controller fails, then neither drive of the mirror set will be accessible. It is wise to install a second controller in the computer so that each disk in a mirror set has its own controller. This arrangement is called a disk duplex. Duplexing is a hardware solution to fault tolerance and should improve performance. It is helpful to add a duplicate power supply as well.

Tape Backup Devices

Cheyenne Software (now owned by Computer Associates) was the first to offer RAID 5 for tape devices. This allows the routine backup window to be met (albeit with 10% - 15% decrease in performance when both drives are treated as one drive) and affords the user the security of archiving the second tape off site. Currently, other backup software products wait for the first tape device to fail, in which case the entire backup job must performed again from the start, requiring as much as double the time that it normally takes to perform the backup.

Because by nature tape devices employ a sequential access method, RAID 5 is an ideal solution for a tape array. The ability to deliver several parallel data streams to an array of tape devices allows for a highly scalable transfer performance. Although tape devices possess very intense error detection and correction algorithms, the user is still left vulnerable to mechanical or tape cartridge failure that could render a critical restore of data unobtainable. A RAID 5-based array of tape devices provides an economical way to protect the backup/restore session data.

RAID level 5 (data striping) is only supported with three or more disk drives in NT, and three tape drives with Cheyenne’s tape software. If one of the drives is destroyed, a complete restore of the data set can still be made from the remaining tapes. If less than three tape drives are used, the data striping will occur, but without fault tolerance. The location of the parity information is moved from drive to drive in a barber-pole fashion. This is extremely important for tape RAID systems, where tape drives compress information.

SUMMARY

When selecting a network’s RAID level, the choice between the RAID 1 and RAID 5 depends on the level of protection desired and an organization’s budget. The major differences between disk mirroring and striping with parity are performance and cost.

Generally, disk mirroring offers better I/O performance. It also has the advantage of being able to mirror the boot or system partition. Disk striping with parity offers better read performance than mirroring; however, the need to calculate parity information requires more system memory and can slow write performance. The cost per megabyte is lower with striping because disk utilization is much greater. For example, if there are four disks in a stripe set with parity, the disk space overhead is 25%, compared to 50% disk space overhead for disk mirroring.

An effective means of achieving fault tolerance is to combine RAID 1 disk mirroring with RAID 5 data striping on the same computer. Consider mirroring the system and boot partitions and protecting the rest of the drive with stripe sets with parity.


Previous Table of Contents Next

footer nav
Use of this site is subject certain Terms & Conditions.
Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.