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Choosing the right RAID level

Recently we had a major computer failure due to a typhoon and water damage. We have sent our disks for data recovery and spent lots of money to recover most of it.

We have now replaced our old server with a Dell PowerEdge 700 with a CERC RAID controller. The server shipped with two drives, and we bought three more of the same. Our data is stored mostly in Exchange Server and the file system for some other database applications (soon to be SQL Server). We are also planning to get an external HDD USB 2.0 of about 200 GB+ for off-site storage in case of typhoon.

My question is, what would be the best array configuration to give us the best fault tolerance, safety and recovery time in case of hardware failure? Each drive is a SATA 120 GB ST3120026AS. The inboard controller is a PERC; the second controller is a CERC 6ch.

I am in favor of RAID 0+1, but have some colleagues that seem to think that RAID-5 would be a wiser choice. Please advise.

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There are a number of factors to consider when choosing the appropriate RAID level for a specific application. Obviously, cost is one of them, but from the onset, the level of data protection and performance are the two main decisive points. The application's read/write ratio will dictate which RAID level is best suited.

RAID 5 is better suited for mostly read-oriented applications. Because RAID 5 has to write parity information for each block of data it writes to the RAID set, it is not well suited for applications that are "write intensive" such as some transactional databases. It should also be noted that RAID 5 alone does not provide multiple-drive failure protection.

RAID 0+1 offers the same level of data protection as RAID 5 with the addition of multiple-drive failure protection. RAID 0+1 is also better suited for write intensive applications, as its mirroring component (RAID 1) does not need to calculate parity information when writing. In addition, its stripping component (RAID 0) offers the same read performance as RAID 5.

Now, looking at your applications: The MS Exchange Information Store is mostly read-oriented, so it is typically better off on a RAID 5. This is the most cost-efficient way to provide data protection while maintaining read performance. However, MS Exchange also uses logs that can get quite busy. These logs being mostly write-oriented, they will perform better on a RAID 0+1 array. You would have to evaluate the read/write ratio for your database application in order to determine the best type of RAID.

Of course, if your applications are not very busy, the performance distinction between RAID 0+1 and RAID 5 becomes much less noticeable and cost can become the driving factor. Ideally, you should consider a combination of both raid technologies if your disk array allows it.

This was first published in October 2004

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