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I am looking at building a NAS. 24 X 500GB 2.5" SATA2 7200RPM 16MB cache drives all in a 2U box with 8GB RAM and 4 CPUs.

I can go two ways:

  1. 1 24+ Port RAID Controller making the whole array one single RAID 10 array
  2. 2 12+ Port RAID Controllers making up two separate RAID10 arrays with only 12 drives per array.

The question is regarding performance. This NAS will has 2 GigE network connections that will be BINDED at the switch to create 2Gig connection for the NAS to the servers.

The data will be for hosting VPSs with typical mysql/apache/smtp/pop3 traffic.

What configuration will be best in terms of performance? Will I get better performance out of two smaller arrays on separate controllers? Or just one big single one?

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How will you fit that many disks in a 2U system? The most I've seen for that form factor of disks is generally 10 per rack unit. Is there a case/server you have in mind specifically? – Luke May 19 '09 at 19:39
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Look at Supermicro SC216 chassis. Note that I am using 2.5" drives, not 3.5". – JonathanLIVE May 19 '09 at 19:55
Thanks. That is definitely slick. – Luke May 19 '09 at 20:00
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3 Answers

up vote 2 down vote accepted

In terms of performance, it comes down to what type of controller you're using. Processor speed, cache, etc. Assuming you're comparing apples with apples and the 24-port controller is faster with twice as much cache there isn't going to be any significant difference with the type of traffic you're talking about.

I'd recommend two smaller arrays for redundancy. If by some miracle one controller starts to fail (i.e. corrupting data to disk, so it won't be immediately obvious) then you haven't lost everything.

Ideally if you've got a really friendly hardware supplier, see if you can get both and do your own tests. Worst case, you might have to pay a restocking fee to return the one you don't use, best case - no charge or just return shipping :)

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After all things considered.. I think I am going to go with the suggestion here to split it into two RAID arrays using two 12 port controllers instead. Talking it over with other tech friends this seems to be the most reasonable, as noted in other commentary, I likely would not be able to get the full power of all drives together over the network. So I am better off for the sake of redundancy to have two separate units. Still going to use the SC216 with 24 2.5" drives in the 2U.. and 2 3ware controllers. – JonathanLIVE May 20 '09 at 16:24
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up vote 1 down vote

A couple of comments;

I'm not entirely sure you'll find a server that takes 24 SFF disks in 2U and gets good enough cooling, I know the HP MSA70 SAS disk enclosure takes 25 SFFs in 2U but that doesn't have much logic behind it to have to cool via the limited air-draw options the disks cause.

That many disks will be able to flood 2Gbps easily in any configuration - I'd be tempted to go RAID6 - you'll get 22 disks worth of storage instead of 12 and it'll easily keep up with its output channel.

Think about a HP DL370 G6 - up to 2 x Nehalem Xeons, 144GB memory, 24 x SFF disks, redundant PSUs in 4U.

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I mentioned in my comment the chassis being used is a Supermicro SC216 which is designed for just this. The heat issue changes when you have 2.5" drives vs. 3.5" The power as well. So are you suggesting I should possibly add 4 nics to the NAS so I have 4Gbps output instead, since you said 2Gbps would fill. I have tried RAID6 configs in the past.. the issue I have with RAID 6 is the performance hit, especially with mysql accesses. I know I am losing 12 disks to the RAID 10 array, but I'm getting far more performance out of it. The HP DL370 G6 would more than triple my budget to build this. – JonathanLIVE May 19 '09 at 21:05
Didn't spot the supermicro ref sorry, still don't like the idea of such little airflow to be honest but that's your call right. I would suggest that you'll be capable of more than 2Gbps with a decent hardware-based RAID controller, whether you go for 4 x 1Gbps or one/two 10Gbps cards depends on what you'll be serving - if it's to only one or two 'client' machines I'd go with the 10Gb option, if it's lots of smaller clients then you should benefit from 4 x 1Gb – Chopper3 May 19 '09 at 21:12
Would you consider each VPS to be a client? Or is it purely the hardware of the Dom0 controller that will impact the difference you just stated? I am expecting to have approximately 100 clients serving from this NAS. The space / power savings are just too great with this configuration for me to pass up. :) – JonathanLIVE May 19 '09 at 21:53
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up vote 0 down vote

Assuming proportional specs (i.e. if the one big controller has N Mb of cache, the two small controllers each have N/2 Mb), the only way you're going to get much benefit out of the split controllers is if your applications are capable of saturating the bus to the controller, or approaching doing so. For 'typical VPS' hosting as your intended application, that seems very unlikely. I'd say you're better off with the one big controller.

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