getting Used space on a newly cloned filer volume

From: Anna M (annpurna@gmail.com)
Date: Wed Feb 06 2008 - 18:17:22 EST

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    If we fetch a volume's space details (using df) on the filer immediately
    after some data is copied/changed on the volume then we do not get accurate
    results.
    Similarly, in cases where we fetch the volume's space details immediately
    after
    its creation from a snapshot of parent volume, we don't get valid results.
    It takes a long time for the "used space" to settle down to a single value
    depending on the amount of data.
    For eg : For a volume of size 10G with used space 9G, the new volume that is
    created takes around 13 minutes to settle down to a consistent used space
    value.
    The pattern that is usually observed is - used_space will start from a very
    high value then it will keep getting lesser till a point in time and after
    that it shoots up again.

    Steps to reproduce this issue:
    Steps to reproduce.

    1. Create a empty volume of size say 10G.
    2. Copy data of size 9G. (Used space is 9G).
    3. Create a snapshot of this volume.
    4. Create a new flexclone using this snapshot and measure
    space stats of the new volume repeatedly with some time lag in between.
       The script I had :
       rsh <filer> vol clone create test_to_check -b <volname> <Snapname>
       rsh <filer> df -k test_to_check
       sleep 1;
       rsh <filer> df -k test_to_check
       sleep 1;
       rsh <filer> df -k test_to_check
       sleep 1;
       rsh <filer> df -k test_to_check
       sleep 1;
       rsh <filer> df -k test_to_check

    In this case, for a parent volume of size 10G with 9G of used space, it
    takes
    around 13min for the used space to settle down for the new volume.

    While cloning a volume of size 310GB, i saw that used_space fluctuted from
    385GB -> 310 GB before it started shooting up again.

    How can I find out the actual used space on this newly created volume? Any
    insight in to this issue will be really useful.

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