Mdadm Bitmap

Discussion in 'Storage & Backup' started by kripz, Apr 14, 2010.

  1. kripz

    kripz Member

    Joined:
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    Messages:
    2,825
    Location:
    Near Frankston
    Just thought id post my results here as there's not much information regarding this feature.

    From the limited information I've read about bitmaps, mdadm records that it is going to write to the array, writes the data and then delete the record if successful. If something happens during the write (eg. power outage), it will need to resync which takes many hours but with the log it just checks if it was doing a write and only fixes that section (or just loose the whole array?).

    There's a total of one single post (IIRC) of performance tests (which were crude) claiming 30% loss if the bitmap was internal and 3% if it was on a disk external to the array.

    I've found that you can enable/disable bitmaps on the fly without rebuilding/resyncing using --grow which is very handy.

    Code:
    mdadm /dev/md0 -G -b internal
    Code:
    mdadm /dev/md0 -G -b /location/of/external/bitmap
    Code:
    mdadm /dev/md0 -G -b none
    Here's my test results on a 7 disk RAID 5 array:

    Code:
    bonnie++ -n 0 -u 0 -d /path/to/folder/on/device/to/test/ | bon_csv2html > /var/www/results.html
    No bitmap
    [​IMG]

    Internal
    [​IMG]

    External
    [​IMG]

    And heres the test of my POS IDE hdd that had the external bitmap

    [​IMG]

    Results aren't 100% but i don't have the time to do more.

    I'm not sure how it compares to an expensive RAID card with a battery as backup.

    If anyone has any information regarding this feature please post, unless my google skill sucks i found like 3 or 4 useful websites which only mention the basic concept and what commands to type in to enable it.
     
    Last edited: Apr 14, 2010
  2. OpTiC Moose

    OpTiC Moose Member

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    Location:
    Queensland - Rockhampton
    I thought i might just post saying great post. Its been handy for me as I'd never really heard of them until i used the ubuntu disk gui to create a raid (I've used command line always in the past) and in mdstat it had this stuff about bitmaps.

    Nice that you can turn them on and off ^_^
     
  3. kripz

    kripz Member

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    I've just purchased a WD green 500gb and will be posting results when i transfer everything, hopefully the bitmap will be faster... not that it's what i bought this drive for. Transfer speeds on the IDE drive were just too slow for me.
     
  4. kripz

    kripz Member

    Joined:
    Sep 29, 2004
    Messages:
    2,825
    Location:
    Near Frankston
    OS HDD (500GB SATA WD)
    [​IMG]

    No bitmap
    [​IMG]

    Bitmap
    [​IMG]

    Raid is nearly full, < 500GB left, i would assume the platters outside is being used hence the speed drop since my last runs a month ago. You cant compare speeds directly but should be able to compare % in speed drops between bitmap and no bitmap.

    Surprisingly my OS hdd has very similar speeds to the 10 year old IDE drive (probably shitty ext3).
    Doesnt seem to be a drop at all bitmap or not now though.

    I'll be adding another 2 drives to the RAID and will be converting to RAID 6.
     
    Last edited: May 23, 2010

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