| Article Index |
|---|
| RAID 101 - Introduction to RAID |
| RAID 0 - Striping |
| RAID 1 - Mirroring |
| RAID 3 - Striping with Dedicated Parity |
| RAID 5 - Striping with Distributed Parity |
| Wrap-Up |
| All Pages |
RAID 0 - Striping
RAID 0, or Striping, requires at least 2 disks, but many RAID controllers allow the creation of a RAID 0 set from up to 8 or 16 disks.
The RAID 0 set is created using the same amount of space on each disk, and the total size is calculated as the size on each disk times the number of disks in the set. If we had a 250GB disk, a 320GB disk and a 500GB disk, the RAID 0 set would use 250GB on each disk, and the total size for the virtual disk would be 750GB.
RAID 0 is the only form of RAID that offers performance without any form of data protection. RAID 0 takes the data on the computer and lays it out across the disks so that some parts of the data are on one disk, and some parts are on other disks.
For our examples, we will use a small portion of binary data - a stream of zeroes and ones. We'll use a simple pattern of 3 ones followed by 3 zeroes for simplicity:
111000111000111000111000111000111000111000111000
Let's store our data on a RAID 0 set of 4 disks. First, we break up the data into smaller chunks, called stripes, which are always sized as blocks of Kilobytes (210 or 1024 bytes) For our example, we'll use stripes of 16 digits each instead:
1110001110001110
0011100011100011
1000111000111000
Now, we divide each stripe into equal parts, and each part is stored on a different disk, one at a time:
| Disk 1 |
Disk 2 |
Disk 3 |
DIsk 4 |
 |
 |
 |
|
Now let's look at what happens if we lose one of the disks, in this case disk 2, and read our data back from the remaining good disks:
| Disk 1 |
Disk 2 |
Disk 3 |
DIsk 4 |
| |
 |
|
|
When we read our data we get "111010001110001111100011100000111000" - which is not the right data! We're missing the second chunk of each stripe, amounting to 1/4 of the data. One whole disk's worth of data is simply lost, and as a result all the data on the RAID 0 set is gone.
In summary, RAID 0 offers performance and size, but cannot protect the data from the failure of a disk - hardly redundant, in spite of the name.
Where Does RAID 0 Make Sense
RAID 0 has it's place, perhaps surprising when all things are considered.
RAID 0 is perfect for temporary files, especially the large temporary files created when editing audio and video. It also has a place in corporate systems where performance is more important than safety of the data, although such configurations are rare.
Let's move on and take a look at RAID 1, or as it's sometimes known, mirroring.
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|192.101.136.xxx |2009-05-28 06:46:55 doublemint - homebuilt NAS ownerI'd also like to know what the recovery process is like, especially if the RAID controller fails. Can a failed RAID controller be replaced with one of a different brand and still recover? As a novice, the RAID BIOS is a bit confusing, some pointers would be helpful. I had a motherboard with RAID fail and assumed I could take one of my RAID 1 drives, plug it into another PC and read it however I couldn’t, why?
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|70.17.178.xxx |2009-05-28 13:53:44 zaphod - replyIm pretty sure youd need both HDs for that to work... not just one.
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|134.211.129.xxx |2009-05-29 17:13:22 David Rawling - Your Mileage Will VaryBasically, the answer is no - you need the same model of RAID controller to replace a failed one.
This applies from the virtually-free Intel RAID controllers on the motherboard to the thousand (and multiple-thousand) dollar add-in cards.
Each controller has its own way of marking the disks as being part of a RAID set - the disk "signature". One brand might write "RAID1-1" to the first disk and "RAID1-2" to the second. Others might use numbers with the brand (MYRAID-716825).
I guess it comes back to the same comment I made before. RAID is not backup. It only protects you from disk failure.
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|71.198.81.xxx |2009-06-01 02:54:12 doublemint - homebuilt NAS ownerDavid,
While unfortunate there isn't more standardization, I understand why this might be. I can't however understand why this would apply to RAID 1 (mirroring). Why do these drives need to be treated any different than non-RAID drives? Just write the same data two places rather than just one.
