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Wednesday, September 15, 2010

'Fake' RAID vs. Software RAID


HOWTO: Linux Software Raid using mdadm

1) Introduction:
Recently I went out and bought myself a second hard drive with the purpose of setting myself up a performance raid (raid0). It took me days and a lot of messing about to get sorted, but once I figured out what I was doing I realised that it's actually relatively simple, so I've written this guide to share my experiences I went for raid0, because I'm not too worried about loosing data, but if you wanted to set up a raid 1, raid 5 or any other raid type then a lot of the information here would still apply.
2) 'Fake' raid vs Software raid:
When I bought my motherboard, (The ASRock ConRoeXFire-eSATA2), one of the big selling points was an on board raid, however some research revealed that rather than being a true hardware raid controller, this was in fact more than likely what is know as 'fake' raid. I think wikipedia explains it quite well:
Hybrid RAID implementations have become very popular with the introduction of inexpensive RAID controllers, implemented using a standard disk controller and BIOS (software) extensions to provide the RAID functionality. The operating system requires specialized RAID device drivers that present the array as a single block based logical disk. Since these controllers actually do all calculations in software, not hardware, they are often called "fakeraids", and have almost all the disadvantages of both hardware and software RAID.
After realising this, I spent some time trying to get this fake raid to work - the problem is that although the motherboard came with drivers that let windows see my two 250 GB drives as one large 500 GB raid array, Ubuntu just saw the two separate drives and ignored the 'fake' raid completely. There are ways to get this fake raid working under linux, however if you are presented with this situation then my advice to you is to abandon the onboard raid controller and go for software raid instead. I've seen arguments as to why software raid is faster and more flexible, but I think the best reason is that software raid is far easier to set up!
3) The Basics of Linux Software Raid:
For the basics of raids try looking on Wikipedia again: I don't want to discuss it myself because its been explained many times before by people who are far more qualified to explain it than I am. I will however go over a few things about software raids:
Linux software raid is more flexible than hardware raid or true raid because rather than forming a raid arrays between identical disks, the raid arrays are created between identical partitions
As far as I understand, if you are using hardware raid between (for example) two disks, then you can either create a raid 1 array between those disks, or a raid 0 array. Using software raid however, you could create two sets of identical partitions on the disks, and for a raid 0 array between two of those partitions, and a raid 1 array between the other two. If you wanted to you could probably even create a raid array between two partitions on the same disk! (not that you would want to!)
The process of setting up the a raid array is simple:
  1. Create two identical partitions
  2. Tell the software what the name of the new raid array is going to be, what partitions we are going to use, and what type of array we are creating (raid 0, raid 1 etc...)
Once we have created this array, we then format and mount it in a similar way to the way we would format a partition on a physical disk.
Other links
The Linux Software Raid Howto:
This guide refers to a package "raidtools2" which I couldn't find in the Ubuntu repositories - use mdadm instead, it does the same thing.
Quick HOWTO: Linux Software Raid
Using mdadm to manage Linux Software Raid arrays
Ubuntu Fake Raid HOWTO In the community contributed documentation

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