RAID 0+1/1+0

Initially wheneveer I see RAID 1+0 or 0+1, I think it should have at least 4 drives:
2 striped and mirrored to 2 other drives
2 mirrored and striped to 2 other drives.
 correct me here if I am wrong.

Now I can see in some scenarios(some Servers that have only 2 SCSI Disks) and the array is 0+1.

can someone explain this?

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Hello Jskfan,

Some storage servers and controllers have the option to perform RAID 1+0 hardware with just two disks, that means you'll have the benefits of both options, but using double the space.

The way this works by stripping the date in both disks, but reversed, that means in one disk you'll fill up from Bottom to the Top and in the other from top to bottom, only till half of the disk, and you use the second haf of the disk to mirror the info in the other disk, like:

Disk1                Disk2
___________   ____________
Stripe1a           MirrorStripe1a
Stripe2a           MirrorStripe2a
Stripe3a           MirrorStripe3a
MirrorStripe3b  Stripe3b
MirrroStripe2b  Stripe2b
MirrorStripe1b  Stripe1b

In this way you'll have speed and safety, but wasting half of the capacity of both disks due to the mirroring.

Your understanding of RAID 0+1 and 1+0 is correct.

When a server only has 2 disks and shows that the array is 0+1 (HP Servers with SmartArray controllers do this), it means that the array is RAID 1, but if it was expanded to 4 or more drives then it would be expanded into a RAID 0+1 set. HP is trying to make things simpler, and confusing people in the process. ;)
HP servers with Smart Array controllers actually show RAID 1+0 rather than 0+1 for 2 disks since HP's terminology is the other way around to most of the storage industry. The older Compaq software shows it as 0+1 but HP even went through the old docs and edited them. You'll also see a single disk as RAID 0 under HP's Array Configurtion Utility.

Since the terminology varies it is particularly important to specify whose controller you're talking about to identify the correct algorithm.
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