Guide to RAID - Critique?

[aram]

Hey guys

I'm making a beginners guide to choosing a RAID setup and was hoping I could get some feedback from people as to how easy it is to follow or to pick up on any technical mistakes I may have made. It's aimed at people who know little or nothing about RAID and are not technically minded.

All feedback is welcome (be gentle!)

Cheers

Aram

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RAID at Home


RAID is a way of using 2 or more Hard Disks Drives together to improve their performance and/or reliability. A group of disks using RAID are called an 'array' and must all be of equal size. If your building a computer and plan on buying more than 1 hard disk, than its well worth considering the different types of RAID array too see if any could enhance your build.

Choosing which RAID setup is best for you will depend on whether your priority is to improve speed or reliability. There are a number of factors that are effected by the type of RAID array you chose:

Read speed is the time it takes for a given piece of data to be retrieved from the array.

Write speed is the time it takes for a given piece of data to be stored in the array.

Reliability is the likely hood of a given piece of data being lost due to one or more of the drives failing.

Cost is dependent on how expensive the array will be to provide a given amount of storage space.

Naturally no system will score highly on every factor, but you should be able to find one which is suitable for your build.

Lets take a look at the different types of RAID array:




RAID 0 'The Stripe'

No. of Drives: 2 +

R-speed ***** W-speed***** Reliability * Cost *****

RAID 0 is used to improve performance of both Read and Write speeds using 2 or more HDDs. It does this by splitting the work between the available disks in 'stripes'. So for example, say you have 2 HDD, it will write half the data to one drive and the other half to the second drive. This (theoretically) doubles the performance of both read and write speeds. In reality the average performance gain is less than that, but still marks a significant improvement over a single drive, particularly when moving large amounts of data around.
The downside to this is that if just one of the drives in the array fails, all the data across the whole array will be lost.
This Type of Array is best suited to situations where both read and write speeds are a priority. It should never be used to store important data.

Suitable for:
• Operating System .
• Storage of data that needs to be written and retrieved quickly (eg. HD video that’s being edited).

RAID 1 'The Mirror'

No. of Drives: 2 + (even no. only)

R-speed **** W-speed *** Reliability ***** Cost **

RAID 1 is the simplest type of array and is used to improve reliability. It needs an even number of drives in the array and simply mirrors the data across the Array. Say you were using 2 drives, it would write the same data to both drives at once, giveing you 2 copies of that data. if one drive were to fail, you would still have all the information stored on the other drive. The down side to all of this is that the cost doubles, as you will only have the storage space of one drive. So if you are using 2 x 500gb drives, you would still only have 500gb worth of storage space, as the other 500gb are being used to backup the first.
This type of array is perfect for someone looking to secure their data against HDD failure.

Suitable For:

• Storage of important Data.
• Operating System.

RAID 5 'The Stripe with Parity'

No. of Drives: 3 +

R-speed ***** W-speed * Reliability **** Cost ****

RAID 5 is a slightly more complicated array which combines some benefits from both RAID 1 and 0. It requires 3 or more drives, using the space of one drive to improve the reliability of the array. It does this by writing a given piece of data to any 2 of the drives in the array. If one drive then fails, it can still retrieve all the data that was stored on it from the other drives in the array. The benefit of this over RAID 1 is that you only lose the space of 1 drive, rather than half the total space. So if you were using 4 disks, then with RAID 5 you still have the space of 3, as oppose to 2 with RAID 1.
It does mean however that if 2 drives go at the same time, then all the data stored across the entire array will be lost.

Speed is something of a mixed bag. Read speeds are extremely good as it acts like RAID 0 array, taking parts of the data from different drives at the same time. Write speeds however are poor, because it has to work out which drives to write the copies or ‘parity’ data to.
This type of array would be good for a media or database server. It's not suited to an Operating System install because the write speeds are so low.

Suitable For:

• Storage of important data that needs to be retrieved quickly (eg. A server Database).

(Image to follow)

RAID 10 'The Striped Mirror'

No. of drives: 4+ (even no. only)

R-speed ***** W-speed ***** Reliability ***** Cost *

RAID 10 is a combination of RAID 1 and RAID 0. Essentially it is like having a striped array, which is then mirrored to give improved reliability. The primary benefits to this system are that firstly you get the performance of a RAID 0, with the reliability of a RAID 1. It can even survive multiple disk failures, provided they are not both on the same side of the stripe. The downside to this setup is cost, as it requires at least 4 drives but will only provide the storage space of 2. The data stored on the array will only be lost if 2 drives fail on opposite sides of the mirror.

Suitable For:

• Operating System.
• Storage of important data that needs to be written and retrieved quickly.


(Image to follow)

Once you have decided on they type or types of array that suit your needs, make sure that the motherboard you’re going to be using supports it. It's also worth remembering that you can have multiple types of array on the same motherboard, so for example you could have a small stripe (RAID 0) for your operating system and a large mirror (RAID 1) to store all your personal files.

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[David M]

Great start aram, it looks professional.

The greatest problem I have found with RAID is finding the right drivers and/or installing those drivers. Also be sure to mention the difference between motherboard based RAID and RAID cards.

RAID "10" is more typically called RAID 0+1 or RAID 1+0, depending on what is emphasized.

Not that I am encouraging plagiarism and I am not, but Wikipedia has good information on RAID. You may be able to use Wiki to check for accuracy..although Wiki is never guaranteed to be 100% accurate.

[aram]

thanks David, thats great

With respect to the differences between raid cards and motherboards, is speed the main advantage of a standalone card? I know write speeds on Raid 5 can be improved alot, but how much real world difference would it make to a mirror or stripe?

[David M]

There was a recent article in CPU or Maximum PC magazine comparing RAID cards....forgot which magazine. Anyway, the basic difference between RAID cards and mobo based RAID is that RAID cards are much easier to deal with...basically they are plug and play not involving drivers that need to be installed during the OS installation. Therefore RAID cards are easy to deal with and they are overall faster than mobo based RAID. The downside to RAID cards is they are costly...anywhere from 150 to 400 bucks if I remember right. I prefer mobo based RAID because it is "free"...it comes with the mobo.

How much real world time between Stripe or Mirror? Honestly?...not much! I have another computer with RAID 0+1 and four Raptor drives. Overall it is not worth it in the real time read/write times. Seriously, whats an additional few seconds, if that, given the cost of adding an additional drive? With my recent build I decided that RAID zero was not worth it but RAID 1 was worth it because of the complexity of my system (three partitions-three operating systems) and the hassle of reinstalling all that if my C: drive were to crash.

If I were building a basic computer and wanted to keep things simple, I would not have RAID at all. Generally speaking, the simpler you keep a computer, the less likely it is to fail. Its probably better to get a fast Seagate Barracuda with a 32 mb buffer than to deal with RAID 0...for the typical user.

David

[aram]

thanks for the info mate, thats really useful, Ill make sure it goes into the next draft