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We've always had performance issues with our Xenserver 5.6 and ive asked a question here about it and still didnt manage to get it resolved (http://serverfault.com/questions/231661/xenserver-vms-continous-temporary-lag).

Since this appears to be a storage thing not being able to put up with 10 XP-SP3 and 2 W2k3 VMs, we got one storage blade for each of our BL460 servers to provide with DAS hence each server will have 4 instead of 2 SAS 10k drives.

My questions:

  • Between RAID 10 and RAID 5 on a p400i raid controller, which do you recommend for improved performance? there are some confusing opinions on the net between those 2.

  • Currently (storage blade not installed yet) the p200i raid controller's smartarray on the BL460 allows for creation of a logical drive with RAID 10, and indeed thats the configuration we're using. but how is that possible with only 2 physical drives?

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2 Answers 2

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Raid 10 with 2 drives ends up being a raid 1 (check the "fault tolerance" section here). Raid 10, generally, is a system where every pair of drives you add becomes a mirrored pair, and all the volume data is striped across all the pairs. Since there's no parity to calculate, it's faster than raid 5. for writes. Reads will be the same. If you have an ASIC on your raid card that can calculate parity in-line, you can get almost the same performance on writes with raid 5. Also, as someone else answered, ensure that you have caching enabled, at least for writes.

For performance alone, I'd always recommend raid 10. If you see the data growing and want to delay more disk purchases in the future, raid 5 is a fair trade-off. Raid 5 lets you use the space on all your disks except one. Raid 10 reduces the amount of space you can use by half.

For reliability, raid 10 still has the lead, but not as much as you'd think. If you lose the wrong two drives, a double disk failure can still hose your data and force you to recover from tape. Raid 5 will always fail on a double disk failure, but raid 10 only sometimes fails on a double disk failure.

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thanks Basil i'll go with a 4-disk array of raid 10 for my new configuration. in your opinion, would the current 2-disk raid 10 (raid 1) array be causing the deficiency of performance? –  3a2roub Feb 16 '12 at 15:39
    
It'll be as fast as a single disk for writes, and as fast as two disks for reads. Performance is measured in IO/s and MB/s. The observed numbers will depend heavily on what fraction of your IO is sequential (predictable) versus random. Also whether read-ahead caching and write caching are enabled. Lastly, the spindle speed of the disks you use. –  Basil Feb 16 '12 at 20:03
    
When you say you have bad performance, how are you measuring that? –  Basil Feb 16 '12 at 20:03

try enable write cache in raid controller.

http://www.tricksguide.com/how-to-check-array-accelerator-cache-battery-bbwc-fbwc-hp-server.html

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for the new configuration with 4 disks, write cache will be enabled. unapplicable now with the p200i. i think this was helpful for me: serverfault.com/questions/110006/… –  3a2roub Feb 16 '12 at 14:55

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