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I'm trying to use zerofree, which seems to be the answer for this: How to reduce VM size?

I have successfully umounted /dev/sda1, and zerofree worked fine, releasing ~37gb.

But I can't seem to umount or remount as read only /dev/mapper/mypc-root, it says:

umount: / is busy


mount: / is busy

The commands used were:

$ umount /dev/mapper/mypc-root
$ mount -no remount,ro -t ext2 /dev/mapper/mypc-root /

I'm in single user mode, entered it with 'init 1'.

The guest SO is Ubuntu 12, with encrypted LVM and home.

Any ideas?

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2 Answers 2

/ can't be unmounted from the system, as / is in use (thus the message you see) by the system itself.

You will need to do this procedure from a Live-CD.

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It cannot be unmounted, but it can be remounted read-only if there are no open files in write mode open. –  Michael Mar 27 '13 at 10:56

You could follow this post: http://unix.stackexchange.com/questions/42015/mount-is-busy-when-trying-to-mount-as-read-only-so-that-i-can-run-zerofree

In my case I did not want to worry with killing processes. Since I had two virtual machines in my VirtualBoX I mounted the disk of the VM i needed to compress on my other VM and zerofree it from that machine.

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