qemu-img: document 'info --backing-chain'

[sdk/emulator/qemu.git] / qemu-img.texi

@example
@c man begin SYNOPSIS
usage: qemu-img command [command options]
@c man end
@end example

@c man begin DESCRIPTION
qemu-img allows you to create, convert and modify images offline. It can handle
all image formats supported by QEMU.

@b{Warning:} Never use qemu-img to modify images in use by a running virtual
machine or any other process; this may destroy the image. Also, be aware that
querying an image that is being modified by another process may encounter
inconsistent state.
@c man end

@c man begin OPTIONS

The following commands are supported:

@include qemu-img-cmds.texi

Command parameters:
@table @var
@item filename
 is a disk image filename
@item fmt
is the disk image format. It is guessed automatically in most cases. See below
for a description of the supported disk formats.

@item --backing-chain
will enumerate information about backing files in a disk image chain. Refer
below for further description.

@item size
is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
and T (terabyte, 1024G) are supported.  @code{b} is ignored.

@item output_filename
is the destination disk image filename

@item output_fmt
 is the destination format
@item options
is a comma separated list of format specific options in a
name=value format. Use @code{-o ?} for an overview of the options supported
by the used format or see the format descriptions below for details.


@item -c
indicates that target image must be compressed (qcow format only)
@item -h
with or without a command shows help and lists the supported formats
@item -p
display progress bar (convert and rebase commands only)
@item -S @var{size}
indicates the consecutive number of bytes that must contain only zeros
for qemu-img to create a sparse image during conversion. This value is rounded
down to the nearest 512 bytes. You may use the common size suffixes like
@code{k} for kilobytes.
@item -t @var{cache}
specifies the cache mode that should be used with the (destination) file. See
the documentation of the emulator's @code{-drive cache=...} option for allowed
values.
@end table

Parameters to snapshot subcommand:

@table @option

@item snapshot
is the name of the snapshot to create, apply or delete
@item -a
applies a snapshot (revert disk to saved state)
@item -c
creates a snapshot
@item -d
deletes a snapshot
@item -l
lists all snapshots in the given image
@end table

Command description:

@table @option
@item check [-f @var{fmt}] [-r [leaks | all]] @var{filename}

Perform a consistency check on the disk image @var{filename}.

If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
during the check. @code{-r leaks} repairs only cluster leaks, whereas
@code{-r all} fixes all kinds of errors, with a higher risk of choosing the
wrong fix or hiding corruption that has already occurred.

Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
consistency checks.

@item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]

Create the new disk image @var{filename} of size @var{size} and format
@var{fmt}. Depending on the file format, you can add one or more @var{options}
that enable additional features of this format.

If the option @var{backing_file} is specified, then the image will record
only the differences from @var{backing_file}. No size needs to be specified in
this case. @var{backing_file} will never be modified unless you use the
@code{commit} monitor command (or qemu-img commit).

The size can also be specified using the @var{size} option with @code{-o},
it doesn't need to be specified separately in this case.

@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}

Commit the changes recorded in @var{filename} in its base image.

@item convert [-c] [-p] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}

Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
using format @var{output_fmt}. It can be optionally compressed (@code{-c}
option) or use any format specific options like encryption (@code{-o} option).

Only the formats @code{qcow} and @code{qcow2} support compression. The
compression is read-only. It means that if a compressed sector is
rewritten, then it is rewritten as uncompressed data.

Image conversion is also useful to get smaller image when using a
growable format such as @code{qcow} or @code{cow}: the empty sectors
are detected and suppressed from the destination image.

You can use the @var{backing_file} option to force the output image to be
created as a copy on write image of the specified base image; the
@var{backing_file} should have the same content as the input's base image,
however the path, image format, etc may differ.

@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}

Give information about the disk image @var{filename}. Use it in
particular to know the size reserved on disk which can be different
from the displayed size. If VM snapshots are stored in the disk image,
they are displayed too. The command can output in the format @var{ofmt}
which is either @code{human} or @code{json}.

If a disk image has a backing file chain, information about each disk image in
the chain can be recursively enumerated by using the option @code{--backing-chain}.

For instance, if you have an image chain like:

@example
base.qcow2 <- snap1.qcow2 <- snap2.qcow2
@end example

To enumerate information about each disk image in the above chain, starting from top to base, do:

@example
qemu-img info --backing-chain snap2.qcow2
@end example

@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}

List, apply, create or delete snapshots in image @var{filename}.

@item rebase [-f @var{fmt}] [-t @var{cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}

Changes the backing file of an image. Only the formats @code{qcow2} and
@code{qed} support changing the backing file.

The backing file is changed to @var{backing_file} and (if the image format of
@var{filename} supports this) the backing file format is changed to
@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
string), then the image is rebased onto no backing file (i.e. it will exist
independently of any backing file).

There are two different modes in which @code{rebase} can operate:
@table @option
@item Safe mode
This is the default mode and performs a real rebase operation. The new backing
file may differ from the old one and qemu-img rebase will take care of keeping
the guest-visible content of @var{filename} unchanged.

In order to achieve this, any clusters that differ between @var{backing_file}
and the old backing file of @var{filename} are merged into @var{filename}
before actually changing the backing file.

Note that the safe mode is an expensive operation, comparable to converting
an image. It only works if the old backing file still exists.

@item Unsafe mode
qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
backing file name and format of @var{filename} is changed without any checks
on the file contents. The user must take care of specifying the correct new
backing file, or the guest-visible content of the image will be corrupted.

This mode is useful for renaming or moving the backing file to somewhere else.
It can be used without an accessible old backing file, i.e. you can use it to
fix an image whose backing file has already been moved/renamed.
@end table

You can use @code{rebase} to perform a ``diff'' operation on two
disk images.  This can be useful when you have copied or cloned
a guest, and you want to get back to a thin image on top of a
template or base image.

Say that @code{base.img} has been cloned as @code{modified.img} by
copying it, and that the @code{modified.img} guest has run so there
are now some changes compared to @code{base.img}.  To construct a thin
image called @code{diff.qcow2} that contains just the differences, do:

@example
qemu-img create -f qcow2 -b modified.img diff.qcow2
qemu-img rebase -b base.img diff.qcow2
@end example

At this point, @code{modified.img} can be discarded, since
@code{base.img + diff.qcow2} contains the same information.

@item resize @var{filename} [+ | -]@var{size}

Change the disk image as if it had been created with @var{size}.

Before using this command to shrink a disk image, you MUST use file system and
partitioning tools inside the VM to reduce allocated file systems and partition
sizes accordingly.  Failure to do so will result in data loss!

After using this command to grow a disk image, you must use file system and
partitioning tools inside the VM to actually begin using the new space on the
device.
@end table

Supported image file formats:

@table @option
@item raw

Raw disk image format (default). This format has the advantage of
being simple and easily exportable to all other emulators. If your
file system supports @emph{holes} (for example in ext2 or ext3 on
Linux or NTFS on Windows), then only the written sectors will reserve
space. Use @code{qemu-img info} to know the real size used by the
image or @code{ls -ls} on Unix/Linux.

@item qcow2
QEMU image format, the most versatile format. Use it to have smaller
images (useful if your filesystem does not supports holes, for example
on Windows), optional AES encryption, zlib based compression and
support of multiple VM snapshots.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand)
@item backing_fmt
Image format of the base image
@item encryption
If this option is set to @code{on}, the image is encrypted.

Encryption uses the AES format which is very secure (128 bit keys). Use
a long password (16 characters) to get maximum protection.

@item cluster_size
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
sizes can improve the image file size whereas larger cluster sizes generally
provide better performance.

@item preallocation
Preallocation mode (allowed values: off, metadata). An image with preallocated
metadata is initially larger but can improve performance when the image needs
to grow.

@end table

@item qed
Image format with support for backing files and compact image files (when your
filesystem or transport medium does not support holes).  Good performance due
to less metadata than the more featureful qcow2 format, especially with
cache=writethrough or cache=directsync.  Consider using qcow2 which will soon
have a similar optimization and is most actively developed.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand).
@item backing_fmt
Image file format of backing file (optional).  Useful if the format cannot be
autodetected because it has no header, like some vhd/vpc files.
@item cluster_size
Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller
cluster sizes can improve the image file size whereas larger cluster sizes
generally provide better performance.
@item table_size
Changes the number of clusters per L1/L2 table (must be power-of-2 between 1
and 16).  There is normally no need to change this value but this option can be
used for performance benchmarking.
@end table

@item qcow
Old QEMU image format. Left for compatibility.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand)
@item encryption
If this option is set to @code{on}, the image is encrypted.
@end table

@item cow
User Mode Linux Copy On Write image format. Used to be the only growable
image format in QEMU. It is supported only for compatibility with
previous versions. It does not work on win32.
@item vdi
VirtualBox 1.1 compatible image format.
@item vmdk
VMware 3 and 4 compatible image format.

Supported options:
@table @code
@item backing_fmt
Image format of the base image
@item compat6
Create a VMDK version 6 image (instead of version 4)
@end table

@item vpc
VirtualPC compatible image format (VHD).

@item cloop
Linux Compressed Loop image, useful only to reuse directly compressed
CD-ROM images present for example in the Knoppix CD-ROMs.
@end table


@c man end

@ignore

@setfilename qemu-img
@settitle QEMU disk image utility

@c man begin SEEALSO
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.
@c man end

@c man begin AUTHOR
Fabrice Bellard
@c man end

@end ignore