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How many RAID sets do I need?

I am setting up a 1 TB HP MSA1000 SAN with three servers hooked into it. Would the best practice be to set up three RAID sets, one for each server? Or should I create one large RAID set and share it across all 3 servers?

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There are a couple of different best practices for setting up your HP MSA1000 SAN with three servers. First, I would have a couple of questions to better know your specific environment. How many disk drives and what is their capacity size for your MSA1000 storage device? What are the performance characteristics of the applications running on the servers? Are they heavy read, heavy write, random or sequential I/O, small I/O or large I/O, database, OLTP, office type tools, e-mail, file serving, or Web servers? By understanding the answers to these questions you can then decide on the appropriate RAID configuration and LUN assignments for the different servers. For example, if the servers and their applications are I/O intensive with primarily writes, then you would want to avoid a RAID-5 configuration, or at least for the data that is write intensive.

Generally speaking, assuming that all three servers have similar performance requirements and you do not have very many disk drives, you would create a single RAID group/set, create multiple LUNs, and then assign the LUNs to the various servers. A word of caution: This is a very generic rule of thumb based upon a few assumptions, including very few available disk drives and servers with low performance criteria. If you have enough disk drives, you can create multiple RAID sets and one or more LUNs and map those to the different servers. What you will want to be careful of is balancing the I/Os of the different servers across the different RAID groups and LUNs to avoid contention of the LUNs and disk drives.

Regardless of whether you create one large RAID set or multiple RAID sets, you will need to create at least one LUN per server and more likely multiple LUNs depending upon your application requirements. You might want to also look at the HP StorageWorks Web site if you have not already done so to view some of the MSA1000 configuration and RAID material for specific details.

This was first published in July 2004

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