Re: RAID or NOT to RAID? What's the diff???

On Mon, Aug 13, 2001 at 08:05:42AM -0800, Koivu, Lisa wrote:  

> Admitting ignorance here. What is the difference between Raid 0+1 and Raid
> 1+0 (is this mirroring)? If someone can forward a website that describes
> the difference I would really appreciate it.

0 + 1 or 1 + 0 is determined by which type you lay down first.

RAID 0 is striping.
So in a simple example of 2 disks, when you lay down the initial RAID you create stripes of a given size across both disks. Say you have 2 1GB disks. If your stripe size was 1MB ( ridiculously large ) you would have 2048 stripes, 512k of each stripe would be written across each disk. 2GB of space would be useable. If one disk fails you lose the entire striped set.

RAID 1 is mirroring.
If you were to mirror the same 2 disks then each should be an identical copy of the other. So once data was written to one disk it would also appear on the other. In this setup ( given the same 2 tiny disks ) only 1GB would be useable. This is almost always a 1:1 operation. Generally speaking you wouldn't mirror a disk to more than one disk. If one disk fails the system keeps operating on the remaining disk

RAID 0 + 1 is Mirrored Stripes.
Now, given the above, say you have 6 disks each of 1GB. In the 0+1 scenario you would divide your disks into 2 groups of 3 disks each. Create 2 RAID 0 sets of 3 disks each. Each of these stripes is 3GB.

Here's a bad ASCII drawing:

0 0
| |
0 0
| |
0 0

Now, you lay down RAID 1 across the stripes. This way if a disk in either stripe ( and hence the entire stripe ) fails, the system will continue operating using the remaining stripe. When the failed disk is replaced the system has to resync the entire stripe, potentially 3GB if the stripe is full. Total available space is 3GB

Here's another ASCII drawing trying to picture the above.

0       0
|       |
0 <-1-> 0
|       |
0       0

RAID 1+0 is Striped Mirrors.
So again, 6 disks. This time you mirror first. You divide your 6 disks into groups of 3, and mirror across them.

Again with the ASCII

1-1

1-1

1-1

Now you stripe across the mirrors. Remember the stripes are divided by the number of targets in the stripe, so given 1MB stripes, each disk will contain roughtly 341K. If a disk dies, that disks mirror takes over and assuming the worst case of a full stripe when the failed disk is swapped out, you will have to wait for 1GB to resync. Total available space is 3GB

Last ASCII I swear ;)

1-1
 |
 0
 |
1-1
 |
 0
 |
1-1

Hope this doesn't confuse you more.

-steve

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