Playing with sparse files

Peter McNabb peter at
Fri Mar 11 12:41:06 MST 2011

On Thu, Mar 10, 2011 at 12:09:08PM -0700, Joseph Hall wrote:
>Question 1: Why is my non-sparse file full of zeros, sitting inside
>the filesystem inside the new sparse file, not taking up a lot of

cp creates holes for sparse files from long sequences of 0's.

This leads to a handy trick when you are copying a VM disk image. If the
VM has been running for a while and writing/unlinking files, you can do:

dd if=/dev/zero of=/tmp/foo; rm -f /tmp/foo

Then shutdown the VM, copy the disk image with "cp --sparse=always", and
your new disk image file should be much smaller.


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