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+.TH "ZEROFREE" "8" 
+.SH "NAME" 
+zerofree \(em zero free blocks from ext2, ext3 and ext4 file-systems 
+.SH "SYNOPSIS" 
+.PP 
+\fBzerofree\fR [\fB-n\fP]  [\fB-v\fP]  [\fB-f fillval\fP]  \fIfilesystem\fR 
+.SH "DESCRIPTION" 
+.PP 
+\fBzerofree\fR finds the unallocated, 
+blocks with non-zero value content in an ext2, ext3 or ext4 
+\fIfilesystem\fR (e.g. /dev/hda1) and 
+fills them with zeroes (or another octet of your choice). 
+ 
+ 
+.PP 
+Filling unused areas with zeroes is useful if the device on 
+which this file-system resides is a disk image. In this case, 
+depending on the type of disk image, a secondary utility may be 
+able to reduce the size of the disk image after zerofree has 
+been run.  
+ 
+.PP 
+Filling unused areas may also be useful with solid-state 
+drives (SSDs). On some SSDs, filling blocks with ones (0xFF) 
+is reported to trigger Flash block erasure by the firmware, 
+possibly giving a write performance increase.  
+ 
+.PP 
+The usual way to achieve the same result (zeroing the 
+unallocated blocks) is to run \fBdd\fR (1) to 
+create a file full of zeroes that takes up the entire free 
+space on the drive, and then delete this file. This has many 
+disadvantages, which zerofree alleviates:      
+.IP "   \(bu" 6 
+it is slow; 
+.IP "   \(bu" 6 
+it makes the disk image (temporarily) grow to its maximal 
+extent; 
+.IP "   \(bu" 6 
+it (temporarily) uses all free space on the disk, so other 
+concurrent write actions may fail.  
+ 
+.PP 
+\fIfilesystem\fR has to be unmounted or 
+mounted read-only for \fBzerofree\fR to work. It 
+will exit with an error message if the 
+\fIfilesystem\fR is mounted writable. To 
+remount the root file-system readonly, you can first switch to 
+single user runlevel (\fBtelinit 1\fR) then use 
+\fBmount \-o remount,ro 
+\fIfilesystem\fR\fR.  
+ 
+.PP 
+\fBzerofree\fR has been written to be run 
+from GNU/Linux systems installed as guest OSes inside a virtual 
+machine. In this case, it is typically run from within the guest 
+system, and a utility is then run from the host system to shrink 
+disk image (\fBVBoxManage modifyhd \-\-compact\fR, 
+provided with virtualbox, is able to do that for some disk image 
+formats).  
+ 
+.PP 
+It may however be useful in other situations: for instance 
+it can be used to make it more difficult to retrieve deleted 
+data. Beware that securely deleting sensitive data is not in 
+general an easy task and usually requires writing several times 
+on the deleted blocks.  
+ 
+.SH "OPTIONS" 
+.IP "\fB-n\fP         " 10 
+Perform a dry run  (do not modify the file-system); 
+.IP "\fB-v\fP         " 10 
+Be verbose; 
+.IP "\fB-f value\fP         " 10 
+Specify the octet value to fill empty blocks with (defaults to 
+0). Argument must be within the range 0 to 255. 
+.SH "SEE ALSO" 
+.PP 
+dd (1). 
+.SH "AUTHOR" 
+.PP 
+This manual page was written by Thibaut Paumard <paumard@users.sourceforge.net> for 
+the \fBDebian\fP system (but may be used by others).  Permission is 
+granted to copy, distribute and/or modify this document under 
+the terms of the GNU General Public License, Version 2 or any  
+later version published by the Free Software Foundation. 
+ 
+.PP 
+On Debian systems, the complete text of the GNU General Public 
+License can be found in /usr/share/common-licenses/GPL-2. 
+ 
+.\" created by instant / docbook-to-man, Mon 31 Mar 2014, 15:41