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My client just bought server with 16 SSD drives in it. What would be the best option to go for RAID? I want to use RAID 10 but I was wondering if I should do them in 4 x RAID 10 (4 drives each) or 8 x 2 o 16 x 1. I guess the safest option is 4 x 4 so that there's a lot of possible options to recover from a failure (less percentage of loosing data due to multiple raid's) and quicker rebuilds. However how would it affect performance? Would 16 x 1 in RAID 10 be better then 4 x 4?

Usage will be Hyper-V and databases.

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closed as not constructive by Zoredache, RobM, pauska, Greg Askew, HopelessN00b Jan 15 '13 at 18:34

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How are they to be used? –  Chopper3 Jan 15 '13 at 17:39
    
Hyper-V as far as I know. Maybe some for database but not sure about it. So if you can provide both options before I go there and set it up would be lovely. –  MadBoy Jan 15 '13 at 17:42
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Hyper-V as far as I know. Maybe some for database but not sure about it. for the sake of argument, pretend that knowing beforehand is useful. –  RobM Jan 15 '13 at 18:02
    
RobM well I simply don't know. Client calls me, tells me to come over and I just want to be prepared for anything. I won't have time when I get there to search for advice from people that used that kind of RAID's. You can respond with comment like yours or you could answer. If you want to use it for databases do this, if you want to use it for Hyper-V do this, if you want to mix it do this. Simple as that. –  MadBoy Jan 15 '13 at 18:03
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Short of any specific knowledge, just choose an option that requires the least work. Without a specific workload in mind there really very little you can do to make a rational determination. Even if you qualified and said just just for databases, that simply isn't enough information, since there is a huge amount of variation in how different databases operate. –  Zoredache Jan 15 '13 at 18:09

4 Answers 4

up vote 4 down vote accepted

It's impossible to tell you this without know how each VM and each database behaves. I also have absolutely no idea what kind of storage subsystem these SSD's are connected to.

That being said - high end enterprise SAN storage usually run SSD at small sets of RAID5. Why not RAID10 you ask? Simply because 99.999999% of the worlds IT systems doesn't benefit from such ridiculous levels of IOPS. It's also something that requires very fast controllers with multi-core CPU's and what not.

My suggestion is that you carve up three RAID5 sets of 5 ssd's and leave one for hot-spare.

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Are you saying use the hot-spare in a single array or is there a controller that allows multiple arrays to utilize a sort of global hotspare? –  ErnieTheGeek Jan 15 '13 at 18:21
    
Global hotspare is quite common these days –  pauska Jan 15 '13 at 18:22
    
Thanks for the heads up. –  ErnieTheGeek Jan 15 '13 at 18:24

No difference, seriously. Raid 10 has a very nice layout. A lot depends on your usage, but generally - the safety is likely similar. Rather consider whether a 5 or 6 makes sense - SSD have a TON of iops budget. Depending on controller you are just dead anyway as you can pull too much IOPS.

The client may also have wasted money - I just am getting Adaptec 7805Q raid controllers. Nice thing. They can use SSD as transparent read and write cache.

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I'm aware he most likely wasted money but he's "like" that. He never consults before doing his buys and just calls me to come and set things up. His choice ;-) –  MadBoy Jan 15 '13 at 17:44
    
Well, yeah. just saying. I run a large setup like that too (actually 24 discs, some of the SSD) and I am moving them now to a 7805Q with 1tb cache. –  TomTom Jan 15 '13 at 17:47

I'm not sure the maths are good.

Your chances of loosing two disks in a mirror are equal whether they are part of the same RAID or not.

Unless what you meant is, if you do loose one RAID 10 because of a mirror failure then not all data is lost.

But in that case I would just say that if you loose a disk in a mirror, you shouldn't wait to have it replaced. And that goes back to your chances of loosing both disks at the same time.

So my answer would be to use one RAID 10 with 16 disks.

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Well the math seems right. Having RAID 10 on all 16 drives only means I can max out my luck by loosing 2 drives (one from each part). Chances that I will loose exactly 2 drives from the same raid 10 when separated into 4 pieces are less. I could theoretically loose 4 drives and still be able to survive (of course I got to have lots of luck). –  MadBoy Jan 15 '13 at 18:08
    
Actually no, a RAID 10 with 16 disks means you are striping over 8 mirrors. So you can loose up to 8 disks as long as they are not part of the same mirror. Your assumption would be right if you where using RAID 0+1. –  Alex Jan 15 '13 at 20:29
    
Thanks for the explanation. I was assuming it's 8 drive RAID 1 and mirror of that. That's good to know. Thank you. –  MadBoy Jan 17 '13 at 11:26

If your chassis is full then I would go 14 disks in raid 10 with 2 for hot spares. If your chassis isn't full then I'd say pick up one more drive and have 16 in raid 10 and 1 hotspare.

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