How to zero fill a virtual disk's free space on windows for better compression?
I would like a simple open source tool (or at least free) for that. It should probably write an as big as possible file full of
0and erase it afterwards. Only one pass (this is not for security reasons but for compression, we are backing up virtual machines).
Should run from inside windows and not from a disk.
On Linux I do it like this (as a user):
cd mkdir wipe sudo sfill -f -l -l -z ./wipe/
Edit 1: I decided to use sdelete from the accepted answer. I had a look at the sdelete's help:
C:\WINDOWS\system32>sdelete /? SDelete - Secure Delete v1.51 Copyright (C) 1999-2005 Mark Russinovich Sysinternals - www.sysinternals.com usage: sdelete [-p passes] [-s] [-q] <file or directory> sdelete [-p passes] [-z|-c] [drive letter] -c Zero free space (good for virtual disk optimization) -p passes Specifies number of overwrite passes (default is 1) -q Don't print errors (Quiet) -s Recurse subdirectories -z Clean free space
This is an old version. I used the -c switch from the 2nd invocation and this was quite fast (syntax only valid for older versions before V1.6):
c:\>sdelete -c c: (OUTDATED!)
I have the impression this does what I want. The sdelete tool is easy to use and easy to get.
Edit 2: As scottbb pointed out in his answer below, there was a September 2011 change to the tool (version 1.6) The -c and -z options have changed meanings. The correct usage from 1.6 onwards is
c:\>sdelete -z c:
Edit 3: There is a 2.0 version of sdelete and sdelete64. They appear to be buggy when zeroing. It will appear to be stuck at 100% for extremely long times. Some people have reported 10 - 40 times longer. The older version 1.61 does not have this issue. See https://social.technet.microsoft.com/Forums/en-US/2ffb2539-34ba-4378-aa8a-941d243f117e/sdelete-hangs-at-100?forum=miscutils
Edit 4: Now there's the issue of dynamically allocated virtual disc space. If you have a 100GB disk that is not full and uses only 30GB on the host, zero filling should not increase dramatically the size of the disc because that contradicts the purpose of dynamic allocation. There is an answer for
Oracle VM VirtualBox https://superuser.com/q/907196/44402 - but on other stacks like VMWare, Xen, XenServer, etc., this needs to be answered separately.