Re: Sparse Files and Heal

Pranith Kumar Karampuri <pkarampu@xxxxxxxxxx> · Tue, 18 Nov 2014 20:26:39 +0530



    On 11/18/2014 05:35 PM, Lindsay
      Mathieson wrote:

    
      I have a VM image which is a sparse file -
        512GB allocated, but only 32GB used.
       
       
      root@vnb:~#
          ls -lh /mnt/gluster-brick1/datastore/images/100
      total
          31G
      -rw-------
          2 root root 513G Nov 18 19:57 vm-100-disk-1.qcow2
       
       
      I switched to full sync and rebooted.
       
      heal was started on the image and it seemed
        to be just transfering the full file from node vnb to vng. iftop
        showed bandwidth at 500 Mb/s
       
      Eventually the cumulative transfer got to
        140GB which seemed odd as the real file size was 31G. I logged
        onto the second node (vng) and the *real* file size size was up
        to 191Gb.
       
      It looks like the heal is not handling sparse
        files, rather it is transferring empty bytes to make up the
        allocated size. Thats a serious problem for the common habit of
        over committing your disk space with vm images. Not to mention
        the inefficiency.
    
    Ah! this problem doesn't exist in diff self-heal :-(. Because the
    checksums of the files will match in the sparse regions. In full
    self-heal it just reads from the source file and writes to the sink
    file. What we can change there is if the file is a sparse file and
    the data that is read is all zeros (read will return all zeros as
    data in the sparse region) then read the stale file and compare if
    it is also all zeros. If both are 'zeros' then skip the write. I
    also checked that if the sparse file is created while the other
    brick is down, then also it preserves the holes(i.e. sparse
    regions). This problem only appears when both the files in their
    full size exist on both the bricks and full self-heal is done like
    here :-(.

    
    Thanks for your valuable inputs. So basically you found 2 issues. I
    will raise 2 bugs one for each of the issues you found. I can CC you
    to the bugzilla, so that you can see the update on the bug once it
    is fixed. Do you want to be CCed to the bug?

    
    Pranith

    
      thanks,
       
      -- 
      Lindsay
      

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