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I'm trying to do some RAID planning, and the only RAID systems I've worked with in RAID-1.

I'm going to be setting up a RAID-10 server.

Could it make sense to have the primary hard drive with a 150GB Velo. Raptor 10K Disk, with the others in the RAID 60GB SSD disks?

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I'd rather have the SSD be the OS drive and the 150GB raptors be the data drives, but thats just me. –  SpacemanSpiff Feb 12 '11 at 22:45
    
Can this be done with OpenVZ? –  bear Feb 12 '11 at 22:46

3 Answers 3

All the drives in a RAID set need to be identical to each other. Anything else is either a waste of time and money, or a risk to your data. Actually, it is possible to mix and match but if you do, you typically either 'waste' capacity, cause performance issue (the performance benefits of the faster disks are cancelled out by the slower disks) or you run into weird compatibility issues - which just doesn't cut it on a Server. The gold standard is a bunch of identically specified drives: same capacity, same speed, same manufacturer, same firmware version, different batches (so they don't all die at once if a faulty batch of drives escape.. it does happen sometimes!)

A matching set of Velociraptor drives will be much faster than "normal" SATA drives, for sure. SSD drives will be faster still. You might find this entry on this very site's blog useful for talk and data about RAID and SSD drives. It's written for higher end 'enterprise' storage but still talks usefully about the performance of SSD drives in a RAID config.

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I think by primary drive he means just a single hard disk not in the RAID set. –  Bart Silverstrim Feb 12 '11 at 22:40
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By makes sense, it depends...funny how often that is the answer. Anyway, if all you want redundant is your data, sure it makes sense. But if that non-RAID drive fails, your server will be unavailable, which is kind of against the point of having a server running RAID; RAID is supposed to ensure availability, not a backup. –  Bart Silverstrim Feb 12 '11 at 22:41
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Hi Robert,I do apologise for the confusion. The system I was dealing with referred to drives are primary, secondary and tertiary. Do you think I should go for SATA drives, or 10K Velociraptor drives in the config, in terms of speed. Reliability is a bit off hand, since data is very regularly backed up to another server. –  bear Feb 12 '11 at 22:46
    
ct2k7 ok edited my answer a bit to remove the question (now I understand what you want) and add more details to my answer. Hope that helps. –  RobM Feb 12 '11 at 22:55
    
primary, secondary, and tertiary - meaning, only 3 physical drives total? Can't do a RAID 10 with that.. unless I'm still confused about something.. –  Rex Feb 13 '11 at 5:10

In direct answer to your question, no it would not make sense to have one disk at a different size or speed to others.

RAID will mirror/spread your data across multiple disks to create one logical volume. The upside is that your volume can sustain one or more disk failures (except RAID 0). When the array is created, if you have disks of differing sizes it will only create a volume based on the size of the smallest disk. For example if you create a RAID1 array using a 120GB disk and an 80GB disk, the resulting logical volume will only be 80GB.

If you're looking at RAID10, then your useable capacity will be (n/2)*Smin where n is the total number of disks you are using and Smin is the smallest capacity drive.

Although the principle isn't the same for drive speed, you will notice an impact if some of your disks are slower than others as the data is mirrored at write time, depending on your controller.

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All RAID drives need to be the same capacity.

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well,no - not really. Most decent RAID controllers can handle drives of different capacity. However, it will be the "least common denominator" of the drives - meaning it will treat all drives as the size of the smallest drive in the set so space gets "lost". –  Rex Feb 13 '11 at 5:07
    
You should still make all the drives the same capacity. –  Victor Feb 13 '11 at 16:04

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