virt-sparsify(1) Virtualization Support virt-sparsify(1)
virt-sparsify - Make a virtual machine disk sparse
virt-sparsify [--options] indisk outdisk
Virt-sparsify is a tool which can make a virtual machine disk (or any disk image) sparse
a.k.a. thin-provisioned. This means that free space within the disk image can be
converted back to free space on the host.
Virt-sparsify can locate and sparsify free space in most filesystems (eg. ext2/3/4, btrfs,
NTFS, etc.), and also in LVM physical volumes.
Virt-sparsify can also convert between some disk formats, for example converting a raw
disk image to a thin-provisioned qcow2 image.
Virt-sparsify can operate on any disk image, not just ones from virtual machines. However
if a virtual machine has multiple disks and uses volume management, then virt-sparsify
will work but not be very effective (http://bugzilla.redhat.com/887826).
IMPORTANT NOTE ABOUT SPARSE OUTPUT IMAGES
If the input is raw, then the default output is raw sparse. You must check the output
size using a tool that understands sparseness such as "du -sh". It can make a huge
$ ls -lh test1.img
-rw-rw-r--. 1 rjones rjones 100M Aug 8 08:08 test1.img
$ du -sh test1.img
(Compare the apparent size 100M vs the actual size 3.6M)
o Virt-sparsify does not do in-place modifications. It copies from a source image to a
destination image, leaving the source unchanged. Check that the sparsification was
successful before deleting the source image.
o The virtual machine must be shut down before using this tool.
o Virt-sparsify may require up to 2x the virtual size of the source disk image (1
temporary copy + 1 destination image). This is in the worst case and usually much
less space is required.
o Virt-sparsify cannot resize disk images. To do that, use virt-resize(1).
o Virt-sparsify cannot handle encrypted disks. Libguestfs supports encrypted disks, but
encrypted disks themselves cannot be sparsified.
o Virt-sparsify cannot yet sparsify the space between partitions. Note that this space
is often used for critical items like bootloaders so it's not really unused.
You may also want to read the manual pages for the associated tools virt-filesystems(1)
and virt-df(1) before starting.
Typical usage is:
virt-sparsify indisk outdisk
which copies "indisk" to "outdisk", making the output sparse. "outdisk" is created, or
overwritten if it already exists. The format of the input disk is detected (eg. qcow2)
and the same format is used for the output disk.
To convert between formats, use the --convert option:
virt-sparsify disk.raw --convert qcow2 disk.qcow2
Virt-sparsify tries to zero and sparsify free space on every filesystem it can find within
the source disk image. You can get it to ignore (don't zero free space on) certain
filesystems by doing:
virt-sparsify --ignore /dev/sda1 indisk outdisk
See virt-filesystems(1) to get a list of filesystems within a disk image.
Compress the output file. This only works if the output format is "qcow2".
--convert [other formats]
Use "output-format" as the format for the destination image. If this is not
specified, then the input format is used.
Supported and known-working output formats are: "raw", "qcow2", "vdi".
You can also use any format supported by the qemu-img(1) program, eg. "vmdk", but
support for other formats is reliant on qemu.
Specifying the --convert option is usually a good idea, because then virt-sparsify
doesn't need to try to guess the input format.
For fine-tuning the output format, see: --compress, -o.
Debug garbage collection and memory allocation. This is only useful when debugging
memory problems in virt-sparsify or the OCaml libguestfs bindings.
Specify the format of the input disk image. If this flag is not given then it is
auto-detected from the image itself.
If working with untrusted raw-format guest disk images, you should ensure the format
is always specified.
Ignore the named filesystem. Free space on the filesystem will not be zeroed, but
existing blocks of zeroes will still be sparsified.
In the second form, this ignores the named volume group. Use the volume group name
without the "/dev/" prefix, eg. --ignore vg_foo
You can give this option multiple times.
This option is used to make the output more machine friendly when being parsed by
other programs. See "MACHINE READABLE OUTPUT" below.
Pass -o option(s) to the qemu-img(1) command to fine-tune the output format. Options
available depend on the output format (see --convert) and the installed version of the
You should use -o at most once. To pass multiple options, separate them with commas,
virt-sparsify --convert qcow2 \
-o cluster_size=512,preallocation=metadata ...
This disables progress bars and other unnecessary output.
Enable verbose messages for debugging.
Display version number and exit.
-x Enable tracing of libguestfs API calls.
Zero the contents of the named partition or logical volume in the guest. All data on
the device is lost, but sparsification is excellent! You can give this option
MACHINE READABLE OUTPUT
The --machine-readable option can be used to make the output more machine friendly, which
is useful when calling virt-sparsify from other programs, GUIs etc.
There are two ways to use this option.
Firstly use the option on its own to query the capabilities of the virt-sparsify binary.
Typical output looks like this:
$ virt-sparsify --machine-readable
A list of features is printed, one per line, and the program exits with status 0.
Secondly use the option in conjunction with other options to make the regular program
output more machine friendly.
At the moment this means:
1. Progress bar messages can be parsed from stdout by looking for this regular
2. The calling program should treat messages sent to stdout (except for progress bar
messages) as status messages. They can be logged and/or displayed to the user.
3. The calling program should treat messages sent to stderr as error messages. In
addition, virt-sparsify exits with a non-zero status code if there was a fatal error.
All versions of virt-sparsify have supported the --machine-readable option.
Windows 8 "fast startup" can prevent virt-sparsify from working. See "WINDOWS HIBERNATION
AND WINDOWS 8 FAST STARTUP" in guestfs(3).
This program returns 0 if successful, or non-zero if there was an error.
Location of the temporary directory used for the potentially large temporary overlay
You should ensure there is enough free space in the worst case for a full copy of the
source disk (virtual size), or else set $TMPDIR to point to another directory that has
This defaults to "/tmp".
Note that if $TMPDIR is a tmpfs (eg. if "/tmp" is on tmpfs, or if you use
"TMPDIR=/dev/shm"), tmpfs defaults to a maximum size of half of physical RAM. If
virt-sparsify exceeds this, it will hang. The solution is either to use a real disk,
or to increase the maximum size of the tmpfs mountpoint, eg:
mount -o remount,size=10G /tmp
For other environment variables, see "ENVIRONMENT VARIABLES" in guestfs(3).
virt-filesystems(1), virt-df(1), virt-resize(1), virt-rescue(1), guestfs(3), guestfish(1),
truncate(1), fallocate(1), qemu-img(1), http://libguestfs.org/.
Richard W.M. Jones http://people.redhat.com/~rjones/
Copyright (C) 2011-2012 Red Hat Inc.
This program is free software; you can redistribute it and/or modify it under the terms of
the GNU General Public License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program;
if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301 USA.
To get a list of bugs against libguestfs, use this link:
To report a new bug against libguestfs, use this link:
When reporting a bug, please supply:
o The version of libguestfs.
o Where you got libguestfs (eg. which Linux distro, compiled from source, etc)
o Describe the bug accurately and give a way to reproduce it.
o Run libguestfs-test-tool(1) and paste the complete, unedited output into the bug
libguestfs-1.22.6 2013-08-24 virt-sparsify(1)