Raid Question

Discussion in 'Home Networking' started by Wagg, Mar 10, 2009.

  1. Wagg

    Wagg Guest

    Possibly the wrong groups, but not too sure where to post this
    question. A friend of mine has a Raid array, with three 146gb drives
    in a compaq server, and then three 36gb drives, below that. The drives
    were not re done, as the originals were the 36's, and they were just
    swapped around in the cage they are in, the 146's up front at the top,
    but the old ones at the bottom, still with the OS on them. The new
    drives were configured as extra partitions.

    Now my question is, with the 6 drives, between the partitioning and
    setup (done by a professional company I am told) there is a missing
    amount of 200gb between all the drives. Now I know that you loose a
    certain amount with formatting and all that, but 200gb? Someone I know
    who works in raids and just raids said that it doesnt appear to be
    configured correctly, but without him seeing it (he doesn't live
    anywhere near me or my other friend!) he cant be too sure, so I am
    asking, does this sound right?

    The company who set it up to begin with says the missing part is
    normal, even up to that size. Just all seems a bit off to me.

    TIA

    -Ben
     
    Wagg, Mar 10, 2009
    #1
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  2. What level of RAID is it? Different types "waste" different quantities
    of space.

    In this case, I'd guess from your figures that you have two RAID5
    arrays of three drives each. RAID5 allows you to lose one disk in the
    array without ill effect, and when you replace the failed disk the
    array rebuilds back to fault-tolerant. It does this by using
    equivalent space to one whole disk as "parity" error checking codes.

    Boot array: 3x36, of which a third is used for parity.
    Data array: 3x146, of which a third is used to parity.

    So you have 146+36 gig "wasted" as parity, which is 182gig - quite
    close to the 200gig you say.

    Common RAID types:

    RAID0 (or "not a RAID): 2x36gig disks, gives 2x36gig space as one
    virtual disk, and no safety
    RAID1 (mirror): 2x36gig disks, gives 1x36gig space as one virtual disk
    and 1 disk fail tolerance
    RAID5 (parity): Nx36gig disks, gives (N-1)x36gig space as one virtual
    disk and 1 disk fail tolerance

    Also,
    JBOD (also "not a RAID", Just a Bunch of Disks): N disks of any sizes,
    gives the combined sum of diskspace as one virtual disk with no
    safety.

    Cheers - Jaimie
     
    Jaimie Vandenbergh, Mar 10, 2009
    #2
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  3. Since the minimum number of drives needed for RAID 5 is three, and he
    has three of each drive fitted, that strongly suggests RAID 5 is in use.

    So if they are configured in RAID 5, that's okay. The usable capacity
    for RAID 5 is (number of drives) - 1.

    3 x 36GB in RAID 5 = 2 x 36GB = 72GB

    3 x 146GB in RAID 5 = 2 x 146GB = 292GB

    So where he would expect (3x36) + (3 x 146) = 546GB, he actually has (2
    x 36) + (2 x 146) = 364GB, a difference of 182GB.

    Deduct about another 10% from the usable capacity for formatting and
    filesystem overhead and you have a yield of 364 - (10% of 364) = 327GB
    for file storage.

    Add the differences together: 182 + 36.4 = 218.4GB "missing" (it's not)

    And that's not taking into account the difference between 1GB = 1000Mb
    (what the drive makers say) and 1GB = 1024Mb (what computers say)...
     
    Mike Tomlinson, Mar 10, 2009
    #3
  4. I've been using one for a couple of years now.
    It's not striped, so you'll lose everything from the failed drive
    forward - how fatal that is depends on which disk pops and how
    resilient your filesystem is.
    Here's my use case: I've got a NAS box with 3x1Tb in, RAID5 so 2Tb
    disk space. I like to back it up, because RAID isn't backup of course.

    So it's backed up nightly to a 2Tb JBOD. The 4x500gig that used to be
    in the NAS are now in a four bay USB enclosure (Edgestor DAS400) that
    has a "JBOD" switch on the back.

    Nice and simple, completely transparent to the host (which is the NAS,
    so it needs to be transparent), and without any risk of individual
    folder trees running short of space. And if a disk pops, who cares?
    The original is still RAID5 protected, so bang in a fresh disk,
    resync, done.


    I'd never recommend them for anything else except second or later
    level backups, obviously.

    (previously, when the 4x500gig RAID5 = 1.5Tb were in the NAS, I backed
    that up to a 4x320gig JBOD that was the original content of the NAS -
    recycling at work! I upgraded when the JBOD ran out of space)

    Cheers - Jaimie
     
    Jaimie Vandenbergh, Mar 10, 2009
    #4
  5. That's what I thought you meant, but I decided to cover the stripe
    possibility since a file that's half on a dead drive is clearly half
    dead. When trying to recover stuff, the best bet by far is to leave
    the disks in the JBOD array while you do - otherwise as you suspect
    files split across two good drives will be impossible to reconstitute.

    But you should never put yourself in the situation where your only
    hope is to recover from a broken JBOD. That's as crazy as not having
    backups at all.

    Cheers - Jaimie
     
    Jaimie Vandenbergh, Mar 10, 2009
    #5
  6. Wagg

    Wagg Guest

    I think it is a Raid 5, but will get him to check. Making a bit more
    sense now! I will find out and post back if needed, if that doesnt
    answer it all!

    -Ben
     
    Wagg, Mar 10, 2009
    #6
  7. Wagg

    Clint Sharp Guest

    In message
    Hopefully he will have the HP array configuration utility loaded.
    Opening that up will tell you exactly how the disks are configured and
    make the lights on the disks flash to tell you which disk is in which
    volume/RAID set when you select the controller, RAID set, volume etc...
     
    Clint Sharp, Mar 11, 2009
    #7
  8. Wagg

    Clint Sharp Guest

    I've seen them a few times in commercial settings.
    Not to be used for anything that isn't transient or backed up elsewhere
    but the 'you lose everything' may not actually be that important
    depending on the use it's put to.
    It's large, cheap and relatively fast storage that can be used as a
    buffer for backup purposes or used for replicated systems where there is
    no reason to have multiple redundant RAID sets because all servers
    always have full copies of the data on the individual JBODs.

    Perversely, the more you spend on storage the more likely you are to see
    JBOD arrays.
     
    Clint Sharp, Mar 11, 2009
    #8
  9. Wagg

    Jon Guest

    Depends which RAID mode, i.e. morroring or striped.

    EG 2 80s in a mirror array would read as 80Gb (less a bit for
    formatting). In striping mode would show as about 150Gb.
     
    Jon, Jun 23, 2009
    #9

  10. There are different types of RAID. However, 3 disks typically means RAID
    5.

    In RAID 5 a disks worth of storage in the RAID group is used by the RAID
    to provide redundancy.

    I.e. 3 x 146GB = about 265GB after formatting.

    "loosing" 200Gb on a RAID of 3 x 146 and one of 3 x 36 sounds about
    right.


    BR
    Don C
     
    Donald Campbell, Jun 25, 2009
    #10
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