which RAID level for write only?

Discussion in 'Storage' started by Tester A., Sep 15, 2003.

  1. Tester A.

    Tester A. Guest

    I would like to know which raid level is the best for Write operation.
    I am considering Raid 0, Raid 1 , Raid 0+1, Raid 10, Raid 3, and Raid 5.

    Also, are there any Raid Storage support Raid4, except NetApp?

    Tester A., Sep 15, 2003
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  2. Tester A.

    Tester A. Guest

    Yes, I need redundacy, so RAID 0 is not the option for me.
    Also, the files that are written is email messages backup, which is very
    small. This is the backup server, so not much read access is there, unless
    the main server is broken.
    Raid 3 is better than other levels for write operation?
    Tester A., Sep 15, 2003
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  3. Tester A.

    Malcolm Weir Guest

    RAID-4 buys you (as a consumer) nothing, and its presence in NetApp
    products is an artifact of their development priorities/processes.

    If your I/O operations are many and small, then either RAID-5 or a
    stripe of RAID-1 (sometimes called RAID-10 by the marketroids) is what
    you want. If they are few and large, you want (true) RAID-3. If your
    performance following a drive failure is critical, RAID-3.

    Simple math:

    With N disks, the maximum number of simultaneous write ops is:

    RAID-1: (N/2)
    RAID-3: 1
    RAID-4: 1
    RAID-5: (N-1)/2

    [ Obviously, in the case of RAID-3, the time taken for the write is
    dramatically improved over the time taken by a single disk. In the
    other cases, the write time is (very roughly) somewhere between one
    and two times that of a single disk. ]

    Malcolm Weir, Sep 15, 2003
  4. NetApp isn't really RAID-4 in the normal sense, is it? I was under the
    impression that it was actually handled at the file system level.
    Peter da Silva, Sep 16, 2003
  5. Tester A.

    idunno Guest

    When should one choose RAID 3 over RAID 10? Both perform well.
    idunno, Sep 16, 2003
  6. Tester A.

    Bill Todd Guest

    For small writes, a classic RAID-3 implementation may be slightly faster
    because its spindles are synchronized (and the mirrored spindles in RAID-10
    typically aren't, so the write there will take take the longer of the two
    disk access times). But if the RAID-10 controller has stable write-back
    cache such that it can queue up write requests and execute them in optimal
    order, any such advantage may be reduced or eliminated (as it will be for
    large writes in any event, where the access times become a smaller
    percentage of the overall overhead).

    But that's for purely serial small writes. For multiple small writes
    requested in parallel, RAID-10 may well be able to process at least some of
    them in parallel, whereas RAID-3 will serialize them at the parity disk
    (though should at least be able to queue-optimize their execution).

    RAID-10, of course, provides potentially significantly better *read*
    performance for a given usable capacity. It also provides somewhat better
    availability, since with RAID-3 (or -5) the loss of any two disks results in
    data loss whereas with RAID-10 data is lost only if the two disks happen to
    be partners.

    - bill
    Bill Todd, Sep 29, 2003
  7. Tester A.

    Bill Todd Guest

    Hmmm. Most people with read-dominated workloads would likely consider
    unsynched spindles a *feature* of RAID-10, since they'd get the faster of
    the two potential read options (at least if the array were suave) at only
    small expense in write performance. But if the array allows the option of
    synching the spindles for RAID-10 operation, and if you're performing only
    writes (so that the heads can be presumed to be in the same position on both
    mirror partners before each write request - at least I hope you could
    presume that), then I can't think of any situation off the top of my head in
    which the RAID-10 array wouldn't equal the speed of the RAID-3 array, and
    for parallel write operations it might well be superior (with the large
    write-back cache, even somewhat larger writes might be able to achieve some
    parallelism, though you're still going to have a hard limit of half the
    aggregate streaming bandwidth of the disks - the same value as the limit for
    a RAID-3 array of equal usable capacity, assuming that its parity generator
    can keep up).

    - bill
    Bill Todd, Sep 29, 2003
  8. Tester A.

    idunno Guest


    thanks again
    idunno, Sep 29, 2003
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