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Metalun vs. larger raid group

Posted on 2010-09-01
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I was recently asked a question I cannot answer and I'd like to get some outside input on this.
We have an EMC Clariion environment.

We have a server that requires a single 2tb lun and pretty high IOPS, more than a single 5disk raid5 raidgroup can provide.

I have two choices, either go with a 10disk raid5 raidgroup with a single 2tb lun or go with 2x 5disk raid5 raidgroups and create a 2tb metalun across them.

Both would be utilizing all spindles. Every other raidgroup we have is a 5disk raid5 group. Other than "trying to keep all the raidgroups the same" what other reason do I have to go with one over the other?
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Question by:narcus
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by:narcus
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I'd like to add that I am referring to a striped metalun, not concatenate
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by:Duncan Meyers
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The 10 disk RAID 5 group will be marginally faster, however that depends on your I/O profile. 4+1 and 8+1 RAID groups have an edge in some workloads as they allow the CLARiiON to perform "square" back-end writes - this effect will be most noticeable with a workload that generates heavy sequential writes. However, I recommend configuring the RAID groups as two 4+1 RAID 5 groups and use create a 2TB metaLUN out of two 1TB components as this allows for simpler expansion down the track. On the other hand, you could also consider a 10 disk RAID 1/0 group if you have high performance requirements, providing that gives you enough disk space. A 10 disk RAID 1/0 group of 450GB drives will give you 2016GB usable.
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by:narcus
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Thanks for the answer! We don't have insane IO requirements for this, but it DOES require 2tb which is more than one 4+1 can provide so I was left with the choice between the two. While sucking every bit of IOPS out of the array isn't necessary, I also don't want to waste performance. It sounds like it won't really make a difference with either setup. All of our other raidgroups are 4+1 and so far, keeping it homogeneous is the only compelling reason I have to go with a 4+1 over 8+1.
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Duncan Meyers earned 500 total points
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A 4+1 RAID 5 group also rebuilds faster in case of a catastrophic disk failure - that's probably enough to tip it over to 4+1s.

(Depending on what FLARE code you're running, your array will likely have pro-active hot-sparing where the array kicks in a mirror operation to the hot spare if a drive shows signs of failure. The pro-active hot spare operation typically takes about 30 minutes for a FC drive, compared to about 4 hours for a rebuild from parity, so you are less exposed to double drive failure and performance will not be clobbered)
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by:narcus
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That's a pretty significant reason. So far, out of all my google searching, this has been the first GOOD reason to do one over the other.
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by:Duncan Meyers
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Thanks! Glad I could help.
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