Q

RAID for Web server serving HTML and ASP files

When setting up a dedicated Web server to serve HTML and ASP files, what type of RAID is best?

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I'm setting up a dedicated Web server to serve HTML and ASP files. I've already spec'd out the processor and memory for it, but I can't find any documentation recommending what level of RAID to use. I know I need redundancy, but I'm unsure how the different types will affect performance. What type of RAID is best?

The short answer: Use hardware-based RAID 1 or RAID 5, depending on your storage space requirements.

The long answer: Systems design involves balancing several different priorities: cost, performance, scalability and reliability are the most important. Because cost is a factor, the goal of systems design must be to meet current and future needs at the lowest price possible. The RAID level you select affects the other priorities as well. Understanding how the Web server's storage affects these priorities, as well as understanding the properties of different types of RAID, will show you the correct answer.

Performance requirements for a Web server's disk storage tend to be very low. Web servers almost always bottleneck on either network bandwidth or CPU utilization. Disk I/O performance won't have an affect on the Web server performance as long as it doesn't become a bottleneck before either of these other factors. For example, if you host your site on your own premises and have a T1 connection to the Internet, then the maximum bandwidth the site could produce would be about 1.4 Mbps. This is far less than what the disk 

I/O subsystem is capable of producing. In fact, even clients accessing the site over 100 Mbps switched network will bottleneck on network performance before disk I/O becomes a limiting factoreven if your Web server has just a single IDE drive. Web servers cache commonly accessed files, greatly reducing disk I/O, and essentially eliminating the possibility that disk performance will affect your end-users experience.

So, speed isn't a factor because whatever RAID level we pick it'll serve data faster than the network can accept it. We don't need to pick the fastest RAID levels, allowing us to eliminate RAID 0 and RAID 0+1, which are performance optimized. This leaves us to choose between RAID 1 and RAID 5.

This leaves us to consider the scalability and reliability requirement. You already know you need redundancy, which would have eliminated RAID 0. But both RAID 1 and RAID 5 provide redundancy, so they're both equally acceptable.

Therefore, scalability becomes the deciding factor. If you don't anticipate ever needing more than 36G bytes of storage on your Web server, the cheapest solution is to use two 36G byte drives in a RAID 1 mirror. If you need to scale to more than two drives, you're better off using a RAID 5 array. RAID 5 provides redundancy, and is optimized to provide the highest storage capacity possible. So, as the short answer said, you should use hardware-based RAID 1 or RAID 5, depending on your storage space requirements.

This was first published in January 2001

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