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So, I have a Dell R710 with PERC 6/i Integrated and 6 450Gb Seagate 15k SAS disks in RAID10, I have 30 Xen vps working on it. Now I need to deploy second server with same hardware for same tasks and I want to figure out maybe it's a good idea to use RAID5 instead of RAID10 because we have a lot of "free" memory on first server and not so much "free space". How do I find out disks performance on first server and find out could I move it to RAID5 without slowing down of whole system?

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You should stay with RAID 10.

RAID 5 (or any variant thereof, 50, 6, 7, etc.) is always slower, often significantly so.

Mirroring is always best for performance, often significantly so. And since disks are cheap buy more/larger up to capacity. If you need added disk redundancy that you might get with dual or triple parity, instead add hot spares.

In other words, it's a complete waste of money to buy a super fast multi-core multi-processor system with gobs of RAM if you're just going to cheap out and destroy performance by using RAID 5.

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For a quick idea of the raw throughput you can use:

hdparm -tT /dev/sda; hdparm -tT /dev/sda; hdparm -tT /dev/sda

But some more "real-world" test would be better. you can use bonnie++, iozone, or better yet test your application.

I wouldn't recommend raid5.

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