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. If a virtual machine has more than one attached disk, you must
sparsify each one separately.
- 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
- The virtual machine must be shut down before using
- 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.
- Virt-sparsify cannot resize disk images. To do that, use
- Virt-sparsify cannot handle encrypted disks. Libguestfs
supports encrypted disks, but encrypted disks themselves cannot be
- 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
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
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
to get a list of filesystems within a disk image.
- Display help.
- Compress the output file. This only works if the
output format is "qcow2".
- --convert raw
- --convert qcow2
- --convert [other formats]
- Use "output-format" as the format for the
destination image. If this is not specified, then the input format is
Supported and known-working output formats are: "raw",
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
- --format raw
- --format qcow2
- 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 filesystem
- --ignore volgroup
- Ignore the named filesystem. Free space on the filesystem
will not be zeroed, but existing blocks of zeroes will still be
In the second form, this ignores the named volume group. Use the volume
group name without the "/dev/" prefix, eg. --ignore
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
- -o option[,option,...]
- 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 qemu-img
You should use -o at most once. To pass multiple options, separate
them with commas, eg:
virt-sparsify --convert qcow2 \
-o cluster_size=512,preallocation=metadata ...
- This disables progress bars and other unnecessary
- Enable verbose messages for debugging.
- Display version number and exit.
- Enable tracing of libguestfs API calls.
- --zero partition
- --zero logvol
- 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 multiple times.
MACHINE READABLE OUTPUT¶
option can be used to make the output more machine
friendly, which is useful when calling virt-sparsify from other programs, GUIs
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
Secondly use the option in conjunction with other options to make the regular
program output more machine friendly.
At the moment this means:
- Progress bar messages can be parsed from stdout by looking
for this regular expression:
- 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.
- 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
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 file.
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 enough space.
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
Richard W.M. Jones <http://people.redhat.com/~rjones/
Copyright (C) 2011 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
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.