RAID question: why can't mirroring have advantage of stripping?

Discussion in 'Mac Basics and Help' started by rawdawg, Oct 23, 2009.

  1. rawdawg macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #1
    As you can see I am no programming or systems expert and don't understand the specifics of how RAId works. I do understand the differences in RAID setups though. If stripping increases performance by allowing drive 1 to read block A and then allowing drive 2 to read block B then why can't a mirrored set do the same thing?

    I assume I may be confused about what defines a block or specifics on how data is read in a RAID0 setup but so long as there is (for this example) a block A on drive 1 and a block B on drive 2 than I don't see why software shouldn't be able to read both drives simultaneously as in a stripped set (while also retaining the redundant advantages of mirroring for backup purposes)?

    Can someone please enlighten me? There's got to be a reason no one has done this.
     
  2. robbieduncan Moderator emeritus

    robbieduncan

    Joined:
    Jul 24, 2002
    Location:
    London
    #2
    With mirroring the exact, bit-for-bit, data is on both drives. There is no performance advantage to read the same data twice which is what you would do if you read from both drives in a mirrored RAID set.
     
  3. steve2112 macrumors 68040

    steve2112

    Joined:
    Feb 20, 2009
    Location:
    East of Lyra, Northwest of Pegasus
    #3
    As robbieduncan stated, in a mirrored set(RAID 1), the data is copied bit for bit to a second drive. You trade speed for redundancy. There are actually combinations of the two technologies. You can have RAID 0+1, which is where the data is striped across two drives, then mirrored to another two drives. You could also do RAID 1+0, which would mirror the drives, then stripe it across two other drives, but that seems a bit silly to me.
     
  4. belvdr macrumors 603

    Joined:
    Aug 15, 2005
    #4
    Some array controllers are smart enough to read different data from both drives to speed up read access though.

    It's not silly at all. It's a very good system for standalone boxes that are limited by RAID 1's and RAID 5's write speeds.
     
  5. rawdawg thread starter macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #5
    So, is what you are saying is that stripping more or less doubles the rotational speed (RPM) of your 'hard drive system' by alternating data "blocks" between the two (and halving the data on each drive which accounts for the doubling in speed)? And therefore simply telling two hard drives that aren't stripped to alternate data blocks similarly wouldn't be any faster because at the same speed it would need to read all the data??

    Is this correct? Therefore it should be a no brainer to choose highest rotation speed drives for stripping to get the most benefit.

    Also, I never understood the difference between 0+1 and 10 (or 1+0)...
     
  6. belvdr macrumors 603

    Joined:
    Aug 15, 2005
    #6
    Theoretically, maybe, but there are other variables at stake. In striping, let's say you choose 128KB as a stripe size on a 3 drive RAID 5 array. Then for a 640KB file, it would go:

    Code:
    Drive 1     Drive 2     Drive 3
    128KB       128KB       Parity
    128KB       Parity      128KB
    Parity      128KB
    
    RAID 10 and 0+1 require 4 drives at a minimum. They are called nested RAID levels and work like this:

    RAID 10 = Mirror first, then RAID 0 the mirrors. So, drives 1 & 2 are mirrored and drives 3 & 4 are also mirrored. Then there is a RAID 0 array across both mirrored sets.

    RAID 0+1 = RAID 0 first then mirror the arrays. So, drives 1 & 2 are in a RAID 0 array and drives 3 & 4 are also in a RAID 0 array. Then both RAID 0 arrays are mirrored.

    There's alos RAID 50 which I have never seen in use and requires 6 drives at a minimum. So the first 3 drives are in a RAID 5 array and the next 3 drives are in a RAID 5 array. Then those RAID 5 arrays then have a RAID 0 array applied on top.
     
  7. rawdawg thread starter macrumors 6502

    Joined:
    Jan 7, 2009
    Location:
    Brooklyn
    #7
    Thanks belvder,

    what I don't understand is when to use 10 versus when to use 0+1. But I guess that's an off topic question, just curious.

    Actually what I really want is RAID5 (!!) because you get more drive for your money. But can you do that with a MBP!??? That's the question I can't get answered.

    But regarding my previous post about why mirroring can't have benefits of stripping. Is my post a reasonable explanation?-- so I can drop that dream.

    All these questions are in response to a thread I started here:
    http://forums.macrumors.com/showthread.php?p=8695517#post8695517
    previously here but no one helped http://forums.macrumors.com/showthread.php?p=8695197#post8695197
     

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