At work I used Centos 6.4 to convert a large Subversion repo to Git. This required a VM with a 100Gb disk, more than my old iMac could deal with, to the point where I could not update to newer OS version or even run iTunes to update apps.
[dchwalis@imac ~]$ df -h Filesystem Size Used Avail Capacity iused ifree %iused Mounted on /dev/disk0s2 931Gi 915Gi 16Gi 99% 239830061 4150681 98% /
This article “How can I reduce the size of a VM on disk” started me off. Then I found https://forums.virtualbox.org/viewtopic.php?p=34636&sid=5bbe19387a6797d599beb35ec2e8b96c#p34636. Looks like since I used a large portion of the expandable virtual disk, there was data in it, and could not be reclaimed. I needed to zero out the now-deleted files and then pack the virtual disk image.
[dchwalis@centos ~]$ df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/vg_vmxxfscv62979-lv_home 242G 74G 156G 33% /home
First bust the system down to runlevel 1 to prevent other processes from keeping the disk busy.
sudo telinit 1
Now how to run a tool on a mount that was being used. This is why you should ALWAYS create new disks and mount them instead of expanding the system disk.
(Mount command line missing. It disappeared into runlevel 1)
# mkdir /fred # mount -o remount,rw /dev/mapper/vg_vmxxfscv62979-lv_home /fred
I ran zerofree on the device. That took a long time and my iMac kept sleeping. Not good.
# /usr/bin/zerofree /dev/mapper/vg_vmxxfscv62979-lv_home
Then to have fsck run when the VM started up, I did per “Force fsck on reboot”
# touch /forcefsck # reboot
Then back to to have
[dchwalis@imac ~]$ VBoxManage modifyhd --compact centos-64-x64.vdi 0%...10%...
This step took a while.
[dchwalis@imac ~]$ df -h Filesystem Size Used Avail Capacity iused ifree %iused Mounted on /dev/disk0s2 931Gi 780Gi 151Gi 84% 204440272 39540470 84% /
So 16Gb free to 151Gb free. Looks like it worked. Lets start up the VM… Yay!
 “How can I reduce the size of a VM on disk”