Specific RAID-0 Question...

Discussion in 'Mac Pro' started by rmpstudio, Apr 8, 2009.

  1. rmpstudio macrumors member

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    Mar 15, 2009
    #1
    Is it possible to mirror a RAID-0 array to a single drive?

    For example, I will be setting up 3x1TB on a RAID-0 (striped) array and i want to mirror it to one 1TB drive. Is that possible or do i have to resort to Time Machine ... ugh :(

    and YES i've already STFA! no answers as of the start of this thread

    http://forums.macrumors.com/showthread.php?t=676409&highlight=raid+0+to+mirror

    -edit i assuming Time Machine backs up the "volumes" and not the "drives"
     
  2. cmaier macrumors G3

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    #2
    No, you can not raid a single drive.
     
  3. jons macrumors 6502

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    #3
    I'm not sure I completely understand the question, but RAID setups don't work that way.
     
  4. hayduke macrumors 65816

    hayduke

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    #4
    3x1TB in RAID zero will give you a fast ~3TB disk that has 3x the potential for catastrophic failure.

    You can back this up to another 1TB drive, but only if your RAID0 array doesn't have more than ~1TB of data.

    Does that help?
     
  5. rmpstudio thread starter macrumors member

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    Mar 15, 2009
    #5
    wow... i thought it was pretty obvious and straightforward question but i seem to have gotten three different types of responses.

    That's not what i was asking. I know you can't RAID a single drive. I want to do a mirror RAID of my volume of striped data. Make sense?

    YOu're right about the 3x failure. That's why i'm trying to set up the mirror as well.

    You said "back this up to another 1TB drive". That's what i want to do via a MIRROR of the volume of striped data. Is that possible??????
     
  6. darrenscerri macrumors regular

    darrenscerri

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    #6
    Theoretically you can and it's called Raid 10 (or 01 or 1+0 or 0+1). The Raid 0 drives are seen as a single drive, and obviously you can Raid 1 that drive with another drive. Note that the 1TB will lag behind the Raid 0 drives by a factor of 3.
     
  7. rylin macrumors 6502

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    #7
    Theoretically, yes.

    However, many implementations refuse to run on anything but an even number of disks (disks = x*2, where x is 2..n).
     
  8. darrenscerri macrumors regular

    darrenscerri

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    #8
    True:

     
  9. rmpstudio thread starter macrumors member

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    #9
    Why does that requirement make any difference in my situation?

    What i'm trying to do is a basic RAID-1 with the Volume of striped data (disk #1) and the mirror of that volume (disk #2).

    how am i wrong? i don't own SoftRaid but if anybody does, does it only allow you to select a drive or can you select a volume to mirror?
     
  10. nanofrog macrumors G4

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    May 6, 2008
    #10
    It's a result of the fact you're RAID 0 is constructed of 3 drives. In a mirror, the array can only be as large as the smallest disk used. In your case, no 3TB drives exist, so to use a single drive for the mirror, you'd lose capacity. 1TB's worth at a minimum. To retain the capacity of the stripe, the mirror drive/array needs to be of the same or larger capacity as the stripe.

    So in your case, a second array would be the answer. The easiest way to do it, is RAID 0+1. This would allow you to use the existing array, without losing the data, and no need to reconstruct the data from backups. So you'd need another 3 (1TB) drives, also in a RAID 0, then mirror the two together.

    That said, you'd obtain greater redundancy with RAID 10, but would require data restoration from a backup source. Creating a mirror first, would erase the data during the initialization process. To construct it, you just go in the reverse order. Mirror first, then stripe. Hence 1+0 or 10. It happens to produce decent performance, particularly with writes, as it doesn't use parity. It also elliminates the "write hole" issue with RAID 5 or 6.

    Hope this helps. :)
     
  11. DrawingArt macrumors member

    DrawingArt

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    #11
    I have a question if someone can help me. Don't wont to use new thread so I apologize to rmpstudio for using this one.

    Can it be RAID 0 configured this way?

    HDD*1 - 1TB:
    Partition 1: 360GB (boot and software)
    Partition 2: 640GB (data) - striped into RAID 0 with HDD*2 Partition 1

    HDD*2 - 640GB
    Partition 1: 640GB(data) - striped into RAID 0 with HDD*1 Partition 2
     
  12. matthew.russo macrumors regular

    matthew.russo

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    #12
    In short, It cannot be done. You need 2 physical disks to setup raid 1 or 0. You Cannot setup raid based on partitions.

    With 2 disks you could do Raid 1 or 0, 4 disks would allow you to do raid 5 or 6 which essentially is a combination of raid 1+0.

    With 3 disks you cannot do much with them really in a raid setup. but there are a couple of options.

    Option 1: With 3 disks you can setup a Concatenation. Concatenation of disks is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single virtual disk. It provides no data redundancy. As the name implies, disks are merely concatenated together, end to beginning, so they appear to be a single large disk.

    Option 2: Setup 2 disks in raid 0 (striped) and leave one disk for redundancy with time machine.
     
  13. NoNameBrand macrumors 6502

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    Halifax, Canada
    #13
    If your two volumes were the same size, then some software RAID packages might allow you to mirror a striped array onto a single disk. However - you'd lose all performance benefits of the RAID0 half, because your system will wait for the write to finish on the single disk half, which takes three times longer. If the system splits reads, then your reads will be slower, and if it doesn't split reads, you don't know whether you get the fast volume (striped) or the slow one (single).
     
  14. DrawingArt macrumors member

    DrawingArt

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    #14
    Ok. Gotcha. Thanks. What I'll do is probably then:

    HDD#1 - 1TB:
    Partition 1: 360GB (boot and software)
    Partition 2: 640GB (data)

    HDD#2 - 640GB (data)
    HDD#3 - 640GB (data)

    HDD2+HDD3 in raid 0 for better performance. Time machine and double manual backups of raid 0 to HDD1 partition 2 and external drive.
     
  15. matthew.russo macrumors regular

    matthew.russo

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    #15
    Sounds good. If it were me to get the most out of a raid 0 set, I would put the OS and Software on the raid set.

    ie.

    Array 1 (Raid 0) (boot and software and personal data[documents])
    HDD#0 - 640GB
    HDD#1 - 640GB

    HDD#2 - 1TB:
    Partition 1: 360GB (Time Machine)
    Partition 2: 640GB (Data)

    That still leaves you with plenty of room for data and getting the benefit of raid for your OS rather than data just sitting there.
     
  16. DrawingArt macrumors member

    DrawingArt

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    #16

    Aha, I thought about this option too but I thought that photoshop files on raid HDD's would benefit and not software itself ... hmhm. In your preposition I would leave second disk 1TB since I have TC and 3 external WD's connected to it.

    So the question is, will boot and software on raid0 be much faster then working files on raid0 ? Which would I benefit more from ... Or you maybee mean raid0 setup would have boot, software and data and second 1TB hdd would be data backup ... or data that is not used so often. Hmhm this might be best solution. Thanks. I'll probably go then with 2x 1TB in raid 0 (boot+software+data) and 1x 640gb for backup since I already have it at home brand new one sitting in closet...
     
  17. matthew.russo macrumors regular

    matthew.russo

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    #17
    That would definitely be your best solution. Doing so give you 2TB of fast disk (2 x 1TB raid 0) and 640gb which is ample for a time machine backup.
     
  18. DrawingArt macrumors member

    DrawingArt

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    #18
    Great thanks for help. Now hardware question for raid 0 with 2x1TB.

    I'm getting my Pro with 1TB 7200rpm 32mb hdd in a week so I guess it is good that before I get it buy one more 1TB hdd and put it in before first installation of OSx and create raid 0 on installement. Now is that possible to do with preinstalled OSX that comes on my PRO? For second disk I would go with WD 1TB 7200 rpm 32mb Black edition and I guess that will work perfectly with my new PRO and 1TB disk that comes with?
     
  19. matthew.russo macrumors regular

    matthew.russo

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    #19
    I could be wrong, but every raid set I have created erases any data on the disks. I would strongly recommend creating a raid set based on a new install.
     
  20. DrawingArt macrumors member

    DrawingArt

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    #20
    Ok cool then I'll do that yes. Thnx.
     
  21. Mac Husky macrumors regular

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    #21
    Interesting question. I planned a WD VelociRaptor for system and apps disk so far and another 2 disks (WD RE3) going RAIDO with my data. Always thought, that leaving system and apps alone on one disk is usefull when possible?! The way you are thinking would lead me to buy another WD VelociRaptor for Position 2 and set it in RAID0 with the first one (including system, apps, files/data). Of course fully backuped via Time Machine on another intern and extern disk. Or let the WD VelociRaptor go and buy RE3 as disk 1+2 perhaps 3 additionally?!
     
  22. DrawingArt macrumors member

    DrawingArt

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    #22
    To be honest I don't know. I would like to know exactly same question you are asking. If boots+apps+data is better on raid 0 or boot+apps on 1 hdd and data on raid0. Which is faster ... ?
     
  23. nanofrog macrumors G4

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    May 6, 2008
    #23
    For a 2 drive RAID0, you'd likely be better off keeping the OS + apps separate (single disk), and data/scratch on the array.

    Adding additional drives to the array can change this however, but you'd also want to consider another implementation. Say 1+0 (aka 10), as it can provide some redundancy as well. :)
     
  24. matthew.russo macrumors regular

    matthew.russo

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    #24
    It depends where you want the performance gain, If you wish for your OS and applications to respond faster then use the Raid set for your boot+apps and optional is your data. The raid set for your data will help improve the time it takes to open, save and auto save to the disk.
     

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