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IT and computer power users would not hesitate to use SCSI drives in their own systems if they could afford it. Owning a SCSI RAID is more affordable than one might think.</p> <p align="left">This article will provide a brief overview of SCSI and of RAID. This article will not go in depth about RAID. I will also cover how I built and inexpensive RAID in my system to ensure data integrity.</p> <p align="left"><b>Why Use SCSI?</b></p> <p align="left">Quality is issue number one in my book. There is nothing worse than having to restore a drive. Everything is not backed up and time is lost with a restore. SCSI provides better stability over IDE because SCSI is designed for heavy industrial use. Do not let the MTBF, Mean Time Between Failures, fool you. MTBF on a SCSI drive is based on a 24 hour a day usage. An IDE drive is based on 7 to 12 hour a day usage. Why do manufactures list MTBF like this on drives? IDE is normally used in home computers or office workstations. The stress on the drive is less because it is not accessed like SCSI drives. An IDE drive is not running 24 hours a day with a full load while SCSI drives are in file servers that are drilled 24 hours a day.</p> <p align="left">In case you do not believe this article about SCSI quality, just think of one thing. An IDE drive comes with a 1 year warranty, while a SCSI drive comes with a 3 to 5 year warranty. Manufacturers determine the length of a warranty based on a failure rate of a drive. Why would a SCSI drive have a longer warranty if the drive was the same quality as an IDE drive?</p> <p align="left">SCSI, or Small Computer System Interface, is an ideal choice for computer performance and the reliability of data. IDE drives in a personal computer can bench test a little faster than SCSI. This is quite common in a computer where the drive is not utilized much. For a network file sever or power user that constantly accesses his drive, a SCSI drive makes a big difference in performance.</p> <p align="left">SCSI drives will outperform IDE drives because of a drive rotation speed. Most SCSI drives provide either 10K or 15K RPMs. An IDE drive is normally 7200 or even worse 5400 RPM. Why are RPMs important? The RPM speed measures how fast a disk spins, naturally. The head on a drive must wait for a sector to become positioned below the head before it can be read or written to. This time delay is known as access time. A drive that spins a disk faster has a lower access time.</p> <p align="left">SCSI is also faster because it can transfer more data at a time. This point is not well illustrated with a home PC because it is rare that a significant amount of data will be transferred over the bus at one time. </p> <p align="left">A SCSI controller card is needed to run SCSI drives. On the controller card is RAM which creates a bigger cache for the drive. The bigger cache allows a drive to buffer data, store data in faster memory. A buffer creates a smaller wait time for the system CPU to receive the data it is requesting; thus, improving processing time and system performance.</p> <p align="left">A processor on the card controls data transfers on a SCSI bus. An onboard CPU allows the SCSI card to handle data access. On an IDE system the main CPU must use resources to perform these tasks. Have you have ever burned a CD and noticed that your system becomes bogged down? A SCSI system does not experience the same degradation in performance. Better performance is due to the SCSI processor performing data transfers instead of the system CPU. If nothing else SCSI is a must for CD and DVD burning.</p> <p align="left"><b>RAID:</b></p> <p align="left">I will give a brief overview of RAID, Redundant Array of Inexpensive Disks. RAID provides redundancy in data by duplicating data on to multiple disks. Data on multiple disks provides protection against disk failure. If a disk should fail, the other disks will have the data on them and the RAID controller can rebuild a new drive when it is put in.</p> <blockquote> <p align="left"><strong>RAID 0 (Striping)</strong></p> <p align="left">Raid 0 breaks data is into chunks (stripes) and stores them on multiple hard drives. Improved performance is obtained from reading and writing to the hard drives in the array in parallel. Data is read and written simultaneously from blocks on each hard drive. Multiple applications can read and write to multiple drives at the same time instead waiting for other applications to finish using a single drive. There is no redundancy in the data; therefore, the full capacity of the hard drives are used. </p> <p align="left"><strong>RAID 1 (Mirroring)</strong></p> <p align="left">RAID 1 is a straight mirror of data. Hard drives are split into two groups. The same data is written to each group. hen reading data, data can be pulled off of either drive. This improves read performance because if two applications are seeking data on the hard drive, one application pulls from drive one while the other pulls from drive two. RAID 1 does not improve write performance since both groups must be written to with the same data. Only half the drive size is available in this array but full redundancy is obtained.</p> <p align="left"><strong>RAID 5</strong></p> <p align="left">RAID 5 creates parity data and distributes them across all the hard drives. Three or more drives must be used with RAID 5. If one hard drive fails, the controller will rebuild the drive based on the parity information on all the other drives. One disk size is normally lost with RAID 5, three 36GB drives are added together to make a 72GB redundant drive.</p> </blockquote> <p align="left">IDE can support RAID. IDE RAID normally supports 4 hard drives. SCSI LVD RAID normally supports 15 drives. For the home PC, I doubt anyone will run more than four drives on a RAID. If so, than SCSI is your choice.</p> <p align="left">If you want to learn more about the different RAID levels go <a href="http://www.solumedia.com/raidinfo.htm">here</a>.</p> <p align="left"><a href="atruereview/Articles/scsi2.php"><next page></a><br> </p> <p align="left"> </p> </td> <td background="../Images/sidebackrt.jpg"width="25"> </td> </tr> <tr> <td width="25" background="../Images/botlftcor.jpg" height="23"></td> <td background="../Images/botback.jpg" height="23" width="490"></td> <td width="25" background="../Images/botrtcor.jpg" height="23"></td> <tr> </p> </div> </td> </tr> </table> </div> <div id="footer" class="clr"> <div class="rk-1"> <div class="rk-2"> <div id="the-footer">  </div> </div> </div> </div> </div> </body> </html>