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