Raid 0

[vantim]

I hope this may answer some questions.
Remember, always back up before RAID’ing or attempting any changes to your drives. Setting up RAID will erase any data on your drives!


So, what is RAID 0?
There are many levels of RAID available. The one we’re interested in is RAID 0.
RAID 0 is a method of improving the rate at which we can read/ write data from hard disks.

Firstly - creating RAID will erase any data on the drives, so backup first!

RAID can be setup in Windows (software RAID) or, using a RAID controller (hardware RAID).

Hardware RAID - requires a RAID controller, either onboard or on a RAID card and often, a copy of the RAID drivers on floppy disk.

Software RAID does not require a RAID controller – howto: http://www6.tomshardware.com/howto/...06/raid-02.html

Both kinds of RAID require more then one Hard drive, ideally identical size + type.

When a hardware RAID 0 “array” is constructed, your operating system will view both hard drives (or more) as one, large drive – which will be faster at reading/ writing files between certain sizes (find out why some file sizes run faster then others later).
This is good for speed. But, as there is always a chance that any hard drive can fail, by joining two together, you are marginally increasing the risk of losing data.
However, even though the risk is low, it’s a good idea to get hold of a copy of Norton Ghost or another good drive imaging program – to take a “snapshot” of your drives contents, to restore later in case of any problems. And, there's another advantage to using a drive imaging program - see the final section
Most RAID controllers are accessible in DOS - there’s been a few Q's about this. Dos treats the drive as normal - you can FDISK, FORMAT whatever, just like a normal drive.

To install Win2000/ XP on hardware RAID, you will often need a copy of the RAID driver on floppy disk.
Check with your motherboard or RAID card manufacturer, or other users of the same type.


How does RAID 0 work?
When you use a RAID controller, you can access more then one hard drives at once (lets say 2, for example - but it depends on the controller)
By splitting your data across two or more disks, which can be read to and written to simultaneously, the data rate can be increased – it doesn’t double for two disks, but there is a noticeable improvement if you set it up correctly.
Read performance gets the most gain; write does too, but not to such a large degree.
The gain you get is because the drives are accessed differently to normal.
In RAID 0, the disks are formatted into many stripes of a fixed size – stripe 1 on disk 1, stripe 2 on disk 2, stripe 3 on disk 1, and so on.
The bigger the file you are reading/ writing, the more stripes it covers across both disks. And, this increases speed – while one drive is reading from a stripe, the other is getting in position to read its stripe and so on. “Prefetch” is a good word for this.

http://images.anandtech.com/reviews/...RAID/RAID0.gif

[linked to ANANDTECH, original picture from PROMISE TECHNOLOGY]


What is “stripe size”, and how can it affect the speed of my RAID 0 array?
The drives are “striped” – “stripes” of a certain size are created on both disks, and the size of each stripe is measured in kilobytes – “stripe size”. The speed our RAID array will access files will be different depending on the stripe size, and the size of the file being accessed.

If you have 32KB stripes, and you are writing a 64KB file, the file is split over the two drives - 32KB each. As one drive is accessing its 32KB stripe, the other is seeking the start of its stripe, so it’s ready to read as soon as the first drive has finished. You can read and write the file faster.

However, it depends on how big the files on your hard drive are on average - if the stripe size is bigger then your file, like below:

E.g. say it’s a 32KB stripe, and a 16KB file

The file is small enough to only be written to one stripe, on one drive. This means it can only be accessed at the speed of one drive, not two or more.
Another problem with stripes in general is that each stripe holds at maximum one file – and any extra space in the stripe that isn’t full, gets wasted.
(112KB for every 16KB written to a 128KB stripe, 128-16=112)
(Wastage gets worse if you have huge stripes, and lots of small files)

If the stripe size is smaller then the file, you do gain speed – like the first example “If you have 32KB stripes, and you are writing a 64KB file”…

But if the stripe size is too small, you lose speed due to excessive drive seeking, and CPU overhead rises.

Not very straightforward, is it? But, never mind – the differences in speed aren’t huge. And, it’s pretty easy to find a good stripe size to start with, and experiment with it.
And, the final section details how to experiment with stripe size, without reinstalling your operating system – if you have a good drive-imaging program like Ghost.


How do I find out what to set the “stripe size” value to?
Depends on how big the files you move around most are, and your hardware (in particular, seek time of your hard drive and CPU power)

You may want to accelerate video editing - in this case, go for the largest stripe available, e.g. 128KB.

To accelerate very small file access, use a small stripe size, e.g. 16KB.

To accelerate your operating system and for general use, I suggest 64K or 32KB.

Otherwise, you want stripes 1/2 the size of the most common files I suppose, theoretically anyway…
A way to find the average file size is to use disk defragmenter’s analyse function, and half the “average file size”.
Stripe size starts at 1KB(!) and goes up in doubles, (1, 2, 4, 8, 16, 32, 64, 128, 256 options) for most controllers.
Go for the next size down for stripe size.

This app will calculate the stripe size exactly as mentioned
stripeSizeCalculator.exe

Going for the next size down, not up, helps make sure as much data as possible is striped (and not just written to one drive) – see “What is “stripe size”

My analysis in disk defrag gives an average file size of 228 - half of 228 = 124, so I chose the next size down – 64K as a start point.
However, this method is not 100% accurate to give you an answer – but, it does give you a good start point (if your average file size is 256K, there’s no point in using 16K for a major boost) - it only takes a few big files to "skew" the results. I think use the average file size as a guide, and try the two or three stripe sizes AROUND THE MOST LIKELY. I tried 32, then 64, and maybe later 128, experimentation guided by the average file size
Experimenting is best - it also depends on your hard drives (seek times in particular), and the controller used.
So, either experiment, or look for test results from someone with as close a system to yours as possible - drive controller/ drive.
As a rough guide, 1/2 the average file size.


How can I experiment with stripe sizes, without reinstalling my O/ S?
Install windows onto your RAID drive, and then use Norton Ghost or a known good drive imaging program to make an image file, or "snapshot" of your drive's contents.
Then change the stripe size - Ghost should be able to restore the image back to your drives, whatever the stripe size is set to (after all, the image isn’t dependant on stripe size).
Recommended - latest version of Ghost.

If you don’t know what size to use, use the default one and use Ghost as above to experiment with different sizes.

Note - I found a bug in Ghost, after selecting "backup" in Windows, it reboots into DOS mode to make the backup and freezes on my system.

When/ if it freezes, reset and boot into DOS with a startup disk, and run "autoexec" from the DOS prompt. This loads Ghost, and backs up