Shrink a thin-provisioned VMDK

If you’ve thin-provisioned a VMDK under ESXi and need to reduce it for whatever reason, the official VMware documentation suggest to migrate the VM to another datastore using VMware converter which is not always practical, thankfully an alternative exists.

If you have enabled Change Block Tracking (CBT) be sure to disable it by adjusting the ctkEnabled option on the virtual machine and consolidating disks before you begin.

To reclaim space you need to fill all unallocated space with zeros. On Windows you can use SDelete or the following command on Linux:

cat /dev/zero > zero.dat;sync;sleep 1;sync;rm -f zero.dat

Once the space has been filled with zeros you can shrink the partition as required. I usually use GParted for this. With your partitions shrunk the next step is to reclaim the space, shut-down the VM and SSH into your ESXi host. “CD” to the directory containing the VM and identify the file you need to shrink for example:

# cd /vmfs/volumes/SXi01-local/SRVGEN02
/vmfs/volumes/53930418-064abd7c-45c9-002590dbfde4/SRVGEN02 # ls -lsah
total 119558160
     8 drwxr-xr-x    1 root     root        2.6K Feb 21 11:01 .
  1024 drwxr-xr-t    1 root     root        2.6K Jan 15 13:27 ..
  8192 -rw-------    1 root     root        7.5M Feb 21 11:00 SRVGEN02-000001-ctk.vmdk
  1024 -rw-------    1 root     root      244.0K Feb 21 11:00 SRVGEN02-000001-delta.vmdk
     0 -rw-------    1 root     root         387 Feb 21 11:00 SRVGEN02-000001.vmdk
  8192 -rw-------    1 root     root        7.5M Feb 21 10:59 SRVGEN02-ctk.vmdk
119531520 -rw-------    1 root     root      120.0G Feb 21 10:59 SRVGEN02-flat.vmdk
  1024 -rw-------    1 root     root        8.5K Feb 21 10:59 SRVGEN02.nvram
     0 -rw-------    1 root     root         589 Feb 21 10:59 SRVGEN02.vmdk
     0 -rw-r--r--    1 root     root          77 Feb 21 11:01 SRVGEN02.vmsd
     8 -rwxr-xr-x    1 root     root        3.0K Feb 21 11:00 SRVGEN02.vmx
     0 -rw-r--r--    1 root     root         263 Jan 17 13:55 SRVGEN02.vmxf
  1024 -rw-r--r--    1 root     root      353.9K Feb 21 10:52 vmware-10.log
  1024 -rw-r--r--    1 root     root      182.9K Feb 21 10:59 vmware-11.log
  1024 -rw-r--r--    1 root     root      182.2K Feb 19 18:53 vmware-6.log
  1024 -rw-r--r--    1 root     root      182.3K Feb 19 19:01 vmware-7.log
  1024 -rw-r--r--    1 root     root      182.3K Feb 19 19:10 vmware-8.log
  1024 -rw-r--r--    1 root     root      183.2K Feb 21 10:46 vmware-9.log
  1024 -rw-r--r--    1 root     root      104.2K Feb 21 10:59 vmware.log

Next, run “vmkfstools –punchzero DISK_NAME.vmdk” to actually shrink the file. How long this takes will depend on the underlying storage, on a reasonably fast SSD this took less than ten minutes for me to shrink 110GB:

/vmfs/volumes/53930418-064abd7c-45c9-002590dbfde4/SRVGEN02 # vmkfstools --punchzero SRVGEN02.vmdk
vmfsDisk: 1, rdmDisk: 0, blockSize: 1048576
Hole Punching: 100% done.

Published by

Dave Hope

Dave works in IT for a leading UK based retirement developer, in his spare time he enjoys tinkering with technology and rock climbing.

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