RAID 0+1/1+0

Posted on 2006-04-16
Last Modified: 2010-04-03
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?

Question by:jskfan
    LVL 15

    Assisted Solution

    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. ;)
    LVL 3

    Accepted Solution

    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.

    LVL 9

    Assisted Solution

    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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