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The Pros and Cons of RAID Level 1

2 April 2002

"To be or not to be, that is the question"...oops, wrong question. While we may not be facing the same challenges Hamlet faced, choosing whether to climb the mountain called RAID can still be vexing. Once only a luxury for the server crowd, "Lite" versions of IDE RAID are now available to the masses.

For many years, I used tape drives to backup my PC. However, as time went by, I would have to keep buying larger tape drives to keep up with the sheer volume of data on my hard drive. Eventually, my diligent backing up slacked off as more and more tapes were required to complete a single backup. Occasionally I would make a Ghost image of my drive, but I did it so infrequently, it was hardly a reliable backup (and DVD-RW/+RW is still too expensive). In the end, I gave up data security altogether by having no backups. As I was planning my next upgrade, I read an article in MaximumPC magazine that recommended buying a motherboard with a built-in hardware RAID controller. Their position was that even if you didn't use the RAID capabilities, you could still split your IDE drives on to separate RAID channels. If you had four IDE devices, each would have its own IDE channel, thus minimizing traffic on each channel. So I paid the extra and purchased the Abit KR7A-RAID motherboard.

Overview

In a basic sense, RAID is simply hard drives working together. RAID stands for Redundant Array of Inexpensive (or sometimes "Independent") Disks. It is a method of combining several hard drives into one logical unit. The levels of RAID are as follows:

RAID 0 RAID 1 RAID 0+1
RAID 5 RAID 0+5
Description
Data striping
(no data protection)
Disk mirroring
RAID 0 and RAID 1 combined
Data striping with distributed parity
RAID 0 and RAID 5 combined
Minimum # of Drives
2
2
4
3
6
Benefit
Highest performance
Data protection through redundancy
Highest performance with data protection
Best balance
of cost/
performance/
data protection
Provides increased fault tolerance

The lite IDE controllers usually provide RAID 0, RAID 1 and RAID 0+1. Two of the newer onboard hardware Ultra133 IDE RAID solutions are the Promise 20271 and the HPT374 controller. Peripheral cards may also offer Level 5 and 0+5 RAID. Finally, there are also third-party and OS software-based RAID solutions.

Since this article is concerned with inexpensive data protection, it will only examine RAID Level 1. In order to satisfy my data protection needs, I had to decide between a weekly backup to a second drive or configure the two drives for RAID 1. What follows is a discussion of some of the Pros and Cons of implementing RAID Level 1.

Pros

Realtime data security - This is the main reason for Level 1 RAID; it provides data protection by duplicating all data from a primary drive on a secondary drive. The second drive is an exact copy of the first drive. Data is written to both drives at the same time. In the event one drive fails, the other drive is an exact duplicate.

No additional partition letters - In Windows Explorer, you do not "see" the second drive's partition letters.

Improved Read Times - During a read, only one drive is typically accessed, but the controller can use both drives to perform two independent accesses. Thus, mirroring improves positioning performance. However, once the data is found, it will be read off one drive; therefore, mirroring will not really improve sequential performance.

Cons

Realtime data security - RAID Level 1's greatest asset is also it's greatest liability. In the event that you screw up something like your Operating System (been there, done that), it's likely that the backup is also screwed up.

Additional wear on second drive - If only weekly backups are performed, the second drive would see only occassional use, but RAID 1 would have the second drive used just as often as the main drive.

Slower write times - During a write, both hard disks are used and performance may be slightly worse than it would be for a single drive.

Less storage space - For example, with two 60GB drives, the total usable storage space is 60GB and not 120GB.

Final Thoughts

I tend to tinker and tweak my Operating System (Windows XP) a lot. A couple of weeks ago I nearly corrupted my system with a bad driver install. Life would have been much easier if I had a backup handy. With RAID Level 1, I would have corrupted the second drive as well as the main drive. For me, a weekly backup is more appropriate. However, if data security is paramount, then RAID Level 1 may be exactly what you need.

Additional Resources -The following are good resources for additional information about RAID and provided background for some of the items above

Adaptec

Storage Review

VIA Arena

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