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  • Latest Post - ‏2013-07-26T12:40:34Z by StorageAdmin
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Pinned topic Can SVC migrate or change a vdisk to Space Efficient vdisk?

‏2010-03-22T20:40:30Z |
How if yes?
Updated on 2010-04-03T00:31:33Z at 2010-04-03T00:31:33Z by SystemAdmin
  • SystemAdmin
    SystemAdmin
    4779 Posts

    Re: Can SVC migrate or change a vdisk to Space Efficient vdisk?

    ‏2010-03-22T20:57:02Z  
    Hi

    The short answer is yes.

    All SVC releases which have Space Efficient Vdisks (SEVs) can use the vdisk mirroring function to mirror from a fully allocated vdisk into a SEV. This is done by adding a space efficient copy to the existing vdisk, waiting for the mirror synchronisation to complete then removing the original copy so that only the space efficient disk is left. However it's not quite that simple...

    Prior to SVC V510 - the space efficient disk will be fully allocated, but you could subsequently expand the vdisk and the expanded area would be space efficient. This isn't very useful to most people - because you don't reclaim any space.

    In SVC V510 or later, the mirroring function will analyse the data being mirrored and if any grain (grain size is a property of the SEV) is all binary 0s then it will not allocate the grain in the space efficient vdisk. However - hosts rarely write binary zeros on a disk so it's not clear how much of the unused disk space will actually be recovered. It is my belief that on *nix operationg systems you should be able to create a new file in the filesystem (using say 60% of the free space) and use something like "dd if=/dev/zero of=myfile" to write binary zeros onto the disk in preparation for the mirroring step. Note - I haven't managed to test this yet to check that it does what I think it does.

    Hope this helps

    Cheers

    Andrew
  • SystemAdmin
    SystemAdmin
    4779 Posts

    Re: Can SVC migrate or change a vdisk to Space Efficient vdisk?

    ‏2010-03-22T21:37:41Z  
    IBM Redbooks have the answer to most SVC questions.

    In your case read section "9.10 Using VDisk Mirroring and Space-Efficient VDisk together"

    Get the book from here:

    http://www.redbooks.ibm.com/redpieces/abstracts/sg246423.html

    Regards, Anthony V
  • cd3lgado
    cd3lgado
    17 Posts

    Re: Can SVC migrate or change a vdisk to Space Efficient vdisk?

    ‏2010-04-02T16:44:56Z  
    Hi

    The short answer is yes.

    All SVC releases which have Space Efficient Vdisks (SEVs) can use the vdisk mirroring function to mirror from a fully allocated vdisk into a SEV. This is done by adding a space efficient copy to the existing vdisk, waiting for the mirror synchronisation to complete then removing the original copy so that only the space efficient disk is left. However it's not quite that simple...

    Prior to SVC V510 - the space efficient disk will be fully allocated, but you could subsequently expand the vdisk and the expanded area would be space efficient. This isn't very useful to most people - because you don't reclaim any space.

    In SVC V510 or later, the mirroring function will analyse the data being mirrored and if any grain (grain size is a property of the SEV) is all binary 0s then it will not allocate the grain in the space efficient vdisk. However - hosts rarely write binary zeros on a disk so it's not clear how much of the unused disk space will actually be recovered. It is my belief that on *nix operationg systems you should be able to create a new file in the filesystem (using say 60% of the free space) and use something like "dd if=/dev/zero of=myfile" to write binary zeros onto the disk in preparation for the mirroring step. Note - I haven't managed to test this yet to check that it does what I think it does.

    Hope this helps

    Cheers

    Andrew
    Hmmm

    Not good. We are in the process of updating from SVC 4.2 to SVC 4.3 and then SVC 5.1 in order to change extend size attribute and use the space efficient volumes to save some space (money).

    Looks like when creating a SEV is a nice idea to format it in order to write zeroes and so being able to use the Space Efficient feature.

    http://www.redbooks.ibm.com/redpieces/abstracts/sg246423.html
  • SystemAdmin
    SystemAdmin
    4779 Posts

    Re: Can SVC migrate or change a vdisk to Space Efficient vdisk?

    ‏2010-04-03T00:31:33Z  
    • cd3lgado
    • ‏2010-04-02T16:44:56Z
    Hmmm

    Not good. We are in the process of updating from SVC 4.2 to SVC 4.3 and then SVC 5.1 in order to change extend size attribute and use the space efficient volumes to save some space (money).

    Looks like when creating a SEV is a nice idea to format it in order to write zeroes and so being able to use the Space Efficient feature.

    http://www.redbooks.ibm.com/redpieces/abstracts/sg246423.html
    Whats not good?

    For zero detect to work it has to find and detect zeros (so it can effectively discard these empty blocks).
    If your disk is filled with trash (deleted files that are removed at the file system layer but still exist as stale unwanted data at the disk level)
    then when you VDisk Mirror to create a space efficient copy, junk data will be copied and NOT discarded (which is undesirable).

    So first we have to remove all the junk by writing zeros over it all, then VDisk mirror to reclaim the space.
    Regards,
    Anthony Vandewerdt