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Old 04-25-2009, 05:13 AM   #1
powadha
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SAS 6iR RAID controller for Dell server


I bought a DELL PowerEdge R200 without the RAID controller. Now I'm wondering if this was the right choice. The controller they offer with the server is the SAS 6iR internal RAID Controller, PCI-Express.
What is your opinion about a RAID controller as such and will it improve performance noticeably?

I'm planning on installing two 640 GB WD drives (they are the same) and do like the RAID0 setup more and more. It can't be done without the controller I think (no mainboard RAID) so otherwise I just have to use two sata drives normally.

Any pointers in having to order the controller afterall?

Regards
 
Old 04-27-2009, 01:14 PM   #2
archangel_617b
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RAID0 isn't really RAID since there's no Redundancy and not suitable for production systems. But if this system is non-production, do whatever you want. The cost of doing AID 0 in software is CPU time and reliability so with a beefy enough CPU, the speed difference may not be very larger but you should be able to find some benchmarks Online for various RAID controllers with an AID 0 configuration. Dell's RAID controllers should be pretty similar to anything on the market.

If you do want RAID (1 or higher) then I would suggest always getting a hardware RAID controller. All our Dell boxen have them and work well. If you've been watching these forums for a while, you'll notice far more people posting problems with software RAID than hardware RAID. The reliability cost of using software is too high.

- Arch
 
Old 04-27-2009, 03:02 PM   #3
farslayer
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Apparently that's an entry level card that is only capable of RAID 0 & 1
http://www.dell.com/downloads/global...0312-Dixit.pdf

A hardware RAID controller can noticibly improve performance over a NON-raid or Software RAID system. But there are other factors to consider when determining the use of RAID.

What is the server to be used for ?
Is Fault tolerance (Your Data) important ?
What is the Spindle speed of the drives being used in the Array ?
How many drives should I use in the array ?

For instance If I were setting up an array to be used for a Database and I was looking for fault tolerance and performance I would opt for a bunch of drives say 10 - 12, with a high spindle speed 10-15K RPM in a RAID 10 configuration.

For a simple array for an OS I might opt for two drives in a RAID 1 Configuration

For File Storage 4 or more Drives in a RAID 5 configuration with a hot-spare.


I might use a mixture of the above on a single server Raid 1 for the OS, and RAID 5 or 10 for the data depending on the servers function.


RAID 0 would be to gain performance (20-30% most likely) without Fault tolerance. if you loose a drive you loose the data, but if that is not important then it is a cost effective way to gain performance. Useful for NON-Linear video editing for instance.


What is your intended use for the array ?
 
Old 04-27-2009, 03:40 PM   #4
johnsfine
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Quote:
Originally Posted by powadha View Post
I bought a DELL PowerEdge R200 without the RAID controller. Now I'm wondering if this was the right choice. The controller they offer with the server is the SAS 6iR internal RAID Controller, PCI-Express.
I don't know the details on that one. Most RAID controllers sold by DELL are "fake raid" and are a total waste of money. The fake raid controller just flags the drives in a way that makes the drivers in Windows (and as I recently, painfully, learned Red Hat) do software raid.

Quote:
It can't be done without the controller I think (no mainboard RAID) so otherwise I just have to use two sata drives normally.
Linux can certainly do software raid. It doesn't need a fake raid controller to tell it to do so.

I don't think there is a significant performance advantage to true hardware raid vs. software raid, especially not for raid0.
 
Old 04-27-2009, 04:44 PM   #5
archangel_617b
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Quote:
Originally Posted by johnsfine View Post
I don't know the details on that one. Most RAID controllers sold by DELL are "fake raid" and are a total waste of money. The fake raid controller just flags the drives in a way that makes the drivers in Windows (and as I recently, painfully, learned Red Hat) do software raid.
What RAID controller did you have this problem with? All the RAID cards from Dell I've dealt with have been hardware RAID. I thought that was the case with anything they were selling with / for their PowerEdge servers...

Thanks,
- Arch
 
Old 04-27-2009, 05:03 PM   #6
farslayer
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That is a Hardware RAID controller with an Integrated ARM I/O processor for RAID operations

Follow the link I posted above.

certainly not fakeraid.
 
Old 04-28-2009, 09:00 AM   #7
johnsfine
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Quote:
Originally Posted by archangel_617b View Post
What RAID controller did you have this problem with? All the RAID cards from Dell I've dealt with have been hardware RAID.
At my current job, all the computers are Dell and most of them are RAID, and so far as I understand, none of them are hardware RAID.

Some use motherboard "raid", some use Promise cards, some use some other brand of raid card. See my recent thread at
http://www.linuxquestions.org/questi...-raid1-720654/
for one example with a Promise card.

Since I haven't used any hardware raid in many years (the last hardware raid I used was PATA not SATA), I'm curious about something:

If you boot up a liveCD (such as Mepis) without any special drivers for raid on a system with hardware raid, what drives do you see:
1) Doesn't work at all because you don't have the drivers?
2) The raid works anyway, because the raid controller makes the multiple SATA drives look like one SATA drive that the software can access without special drivers?
3) Each drive acts like an ordinary individual drive?

Obviously with fake raid it will be (3). Depending on how the fake raid is done, the MBR you see in that mode may or may not be the same sector as the MBR you see when the fake raid is active. So for raid1, you might see two identical drives or you might see garbage. (The Dell systems on which I've used Linux to help diagnose Windows problems have had both versions of that).

For hardware raid, with raid0 or raid1, what exactly does the hardware raid do? The software overhead for raid0 or raid1 is trivial (no XORs). It's hard to imagine what benefit hardware raid provides. I would expect software raid to make better decisions about command queuing and distributing the reads for raid1 than hardware raid could. (I know the Dell software for motherboard raid1 for Windows XP makes absurdly bad decisions about distributing the reads, but I'm hoping/assuming Linux software raid1 wasn't written by idiots).
 
Old 04-28-2009, 10:01 AM   #8
farslayer
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Hardware RAID 0 improves write performance because it can write to multiple drives simultaneously. and since the overhead is handled by the card rather than the CPU it will perform better than software RAID.

Personally I've never run into a Dell server that didn't have actual Hardware RAID. I'm curious as to what you have purchased. all of my Servers have PERC controllers which are typically megaraid or megaraid_sas drivers, which are both in the kernel.

http://linux.dell.com/storage.shtml

Ah there is one controller listed that is fakeraid..
Quote:
The CERC SATA 2S RAID controller is a combination BIOS and driver-based software RAID solution, which uses the system motherboard's SATA controllers. It is not a hardware RAID solution such as the aacraid and megaraid controllers listed above.

Systems with this controller include the PowerEdge 800, SC1425, and 420SC.
I have to wonder if someone at your job added in those Promise cards and other raid controllers after the fact..



I also would rather run pure software RAID than waste my time with fakeraid though..

Last edited by farslayer; 04-28-2009 at 10:03 AM.
 
Old 05-06-2009, 04:47 AM   #9
mthui7
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Quote:
Originally Posted by johnsfine View Post
At my current job, all the computers are Dell and most of them are RAID, and so far as I understand, none of them are hardware RAID.
...
Thank you for your sharing. From the information that I have read so far, this is really a confusing and grey area regarding software/hardware RAID or fake/real RAID. In my previous job experience, I worked with Dell Poweredge 2450 and installed Red Hat 9 (that was around 2-3 years ago) and was really puzzled and confused about this issue. The 3 scenario you mentioned is really useful for analysing the situation. From my past experience with Poweredge 2450, I have had both case 1 and 3. I hate case 1(before I could install any driver) the most and dislike case 3(after I installed any driver that I could get to make it go). As in case 3, if I see multiple SATA drives as ordinary drives, what's the use of the "RAID" card anyway?!. I have never had the luck to see case 2.

I really don't appreciate the whatever fake/software RAID that Dell provides. Before getting any advantages, I got the trouble right from the start. Now I would prefer to get rid of any form of RAID completely. Instead, run a standby server which replicates data and programs on a daily basis(or any other regular time interval). It seems that it puts my mind to rest better at night rather than having whatever "RAID" system.
 
  


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