Zero Out Free Disk Space on Virtual Machines and Compact them Before Backup: Solaris, Linux, Windows, OpenVMS: http://supratim-sanyal.blogspot.com/2016/12/zero-out-free-disk-space-on-virtual.html

zerofile.c-openvms

/* ------------

zerofile.c

Creates a file zerofile.zero in current directoy. This file is as big as disk space allows.
The file zerofile.zero must be deleted after this program exits and before backing up this
SIMH VM to a tarball.

Hopefully size of tarball is smaller because we wrote zeroes to all the empty space on hard
disks.

TESTED ON Compaq C V6.4-005 on OpenVMS VAX V7.3    
To Build ZEROFILE.EXE:
$ CC ZEROFILE.C
$ LINK ZEROFILE.OBJ

Then set default to the drive(s) in which unused space is to be filled with zeroes and run
this program. For example, to fill free space on DUA2: with zeroes (from a privileged 
account) with this program compiled and executable present in DUA1:[TOOLS]ZEROFILE.EXE:

$ SET DEFAULT DUA2:[000000]
$ RUN DUA1:[TOOLS]ZEROFILE
$ DELETE/LOG/CONF ZEROFILE.ZERO;*


(C) 2016 SUPRATIM SANYAL <SUPRATIM AT RISEUP DOT NET>
FREELY USABLE AND REDISTRIBUTABLE UNDER GNU AGPLv3
LICENSE: http://tuklusan.decsystem.org/agpl-3.0-standalone.html
--------- */

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <ctype.h>
#include <ssdef.h>
#include <dvidef.h>
#include <descrip.h>
#include <stsdef.h>
#include <lnmdef.h>
#include <iodef.h>
#include <opcdef.h>
#include <errno.h>

main()
{
        FILE *fp;
        int i=0,j=1;
        const int block=512;
        char *fname="zerofile.zero";
        if(NULL==(fp=fopen(fname,"w")))fprintf(stderr,"Cannot open [%s] for writing: [%d]",fname,errno),exit(1);
        printf("REMEMBER TO DELETE %s WHEN THIS PROGRAM EXITS\n",fname);
        fflush(NULL);
        printf("Press ENTER to create file [%s] with zeroes to fill up hard drive:",fname);
        getchar();
        while(1)
        {
                for(i=0;i<block;i++)if(EOF==fputc('\0',fp))fclose(fp),exit(0);
                printf("%d blocks...",j++);
        }
}