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Fragmentation and Raid

  • 04-10-2008 11:46pm
    #1
    Khannie
    Registered Users, Registered Users 2 Posts: 37,485 ✭✭✭✭


    Is there any point in defragmenting the filesystem on a Raid 5 array?

    I'm thinking: I'm not sure, but I'm questioning it because the individual files are spread over multiple disks anyway, so what's the point in a nice orderly filesystem?

    The specifics of the array are:
    3 x 750G drives
    linux kernel softraid
    XFS filesystem
    About 90% in use.
    About 25% fragmented filesystem. :eek:

    edit: I should say that I don't think the specifics are overly important, but I suppose some people might want them. It's more of a general discussion type question.


Comments

  • #2
    SeanW
    Registered Users, Registered Users 2 Posts: 9,166 ✭✭✭SeanW


    AFAIK the way RAID works is to have a number of similar/identical drives acting in concert. By their very nature, hard drives have data files stored paralell over a number of platters, i.e. parts of the file are read/written in paralell, increasing overall speed. Individual files are not stored solely on a single platter.
    RAIDing has the practical effect of increasing the number of platters in use for all data, as opposed to non-RAID setups where each drive acts independently. So yes, I would imagine there is a point to defragmenting RAID arrays.


  • #3
    Alkers
    Registered Users, Registered Users 2 Posts: 6,908 ✭✭✭Alkers


    Where's the harm?


  • #4
    Khannie
    Registered Users, Registered Users 2 Posts: 37,485 ✭✭✭✭Khannie


    Simona1986 wrote: »
    Where's the harm?

    It's more of a curiosity. I ran the defrag last night anyway. I don't _fully_ understand how the filesystem fragmentation would affect the performance (if at all) given that the 3 disks are being read from anyway. I'm not sure if (for example) all 3 disks are read from in parallel and if so would a defragmented filesystem mean that there is less seek time? That's the theory on a single disk.

    I am certain that there is some parallelisation of reading going on, as I get about 150MB/s read speed from the array and no single disk is capable of that.
    SeanW wrote: »
    Individual files are not stored solely on a single platter.

    That's interesting. I assumed that with a single drive this was the point of a defrag, but of course parallel reads from multiple platters makes more sense. I'm still not getting the bigger picture of filesystem over raid5 though tbh.


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