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@example |
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@c man begin SYNOPSIS |
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usage: qemu-img command [command options] |
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@c man end |
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@end example |
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|
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@c man begin DESCRIPTION |
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qemu-img allows you to create, convert and modify images offline. It can handle |
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all image formats supported by QEMU. |
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|
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@b{Warning:} Never use qemu-img to modify images in use by a running virtual |
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machine or any other process; this may destroy the image. Also, be aware that |
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querying an image that is being modified by another process may encounter |
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inconsistent state. |
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@c man end |
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|
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@c man begin OPTIONS |
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|
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The following commands are supported: |
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|
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@include qemu-img-cmds.texi |
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|
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Command parameters: |
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@table @var |
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@item filename |
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is a disk image filename |
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@item fmt |
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is the disk image format. It is guessed automatically in most cases. See below |
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for a description of the supported disk formats. |
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|
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@item --backing-chain |
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will enumerate information about backing files in a disk image chain. Refer |
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below for further description. |
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|
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@item size |
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is the disk image size in bytes. Optional suffixes @code{k} or @code{K} |
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(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M) |
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and T (terabyte, 1024G) are supported. @code{b} is ignored. |
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|
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@item output_filename |
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is the destination disk image filename |
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|
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@item output_fmt |
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is the destination format |
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@item options |
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is a comma separated list of format specific options in a |
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name=value format. Use @code{-o ?} for an overview of the options supported |
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by the used format or see the format descriptions below for details. |
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@item snapshot_param |
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is param used for internal snapshot, format is |
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'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]' |
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@item snapshot_id_or_name |
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is deprecated, use snapshot_param instead |
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|
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@item -c |
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indicates that target image must be compressed (qcow format only) |
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@item -h |
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with or without a command shows help and lists the supported formats |
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@item -p |
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display progress bar (compare, convert and rebase commands only). |
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If the @var{-p} option is not used for a command that supports it, the |
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progress is reported when the process receives a @code{SIGUSR1} signal. |
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@item -q |
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Quiet mode - do not print any output (except errors). There's no progress bar |
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in case both @var{-q} and @var{-p} options are used. |
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@item -S @var{size} |
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indicates the consecutive number of bytes that must contain only zeros |
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for qemu-img to create a sparse image during conversion. This value is rounded |
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down to the nearest 512 bytes. You may use the common size suffixes like |
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@code{k} for kilobytes. |
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@item -t @var{cache} |
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specifies the cache mode that should be used with the (destination) file. See |
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the documentation of the emulator's @code{-drive cache=...} option for allowed |
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values. |
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@end table |
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|
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Parameters to snapshot subcommand: |
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|
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@table @option |
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|
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@item snapshot |
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is the name of the snapshot to create, apply or delete |
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@item -a |
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applies a snapshot (revert disk to saved state) |
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@item -c |
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creates a snapshot |
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@item -d |
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deletes a snapshot |
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@item -l |
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lists all snapshots in the given image |
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@end table |
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|
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Parameters to compare subcommand: |
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|
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@table @option |
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|
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@item -f |
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First image format |
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@item -F |
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Second image format |
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@item -s |
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Strict mode - fail on on different image size or sector allocation |
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@end table |
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|
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Parameters to convert subcommand: |
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|
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@table @option |
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|
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@item -n |
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Skip the creation of the target volume |
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@end table |
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|
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Command description: |
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|
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@table @option |
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@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] @var{filename} |
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|
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Perform a consistency check on the disk image @var{filename}. The command can |
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output in the format @var{ofmt} which is either @code{human} or @code{json}. |
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|
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If @code{-r} is specified, qemu-img tries to repair any inconsistencies found |
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during the check. @code{-r leaks} repairs only cluster leaks, whereas |
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@code{-r all} fixes all kinds of errors, with a higher risk of choosing the |
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wrong fix or hiding corruption that has already occurred. |
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|
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Only the formats @code{qcow2}, @code{qed} and @code{vdi} support |
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consistency checks. |
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|
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@item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}] |
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|
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Create the new disk image @var{filename} of size @var{size} and format |
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@var{fmt}. Depending on the file format, you can add one or more @var{options} |
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that enable additional features of this format. |
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|
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If the option @var{backing_file} is specified, then the image will record |
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only the differences from @var{backing_file}. No size needs to be specified in |
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this case. @var{backing_file} will never be modified unless you use the |
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@code{commit} monitor command (or qemu-img commit). |
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|
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The size can also be specified using the @var{size} option with @code{-o}, |
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it doesn't need to be specified separately in this case. |
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|
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@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename} |
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|
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Commit the changes recorded in @var{filename} in its base image or backing file. |
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If the backing file is smaller than the snapshot, then the backing file will be |
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resized to be the same size as the snapshot. If the snapshot is smaller than |
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the backing file, the backing file will not be truncated. If you want the |
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backing file to match the size of the smaller snapshot, you can safely truncate |
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it yourself once the commit operation successfully completes. |
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|
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@item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2} |
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|
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Check if two images have the same content. You can compare images with |
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different format or settings. |
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|
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The format is probed unless you specify it by @var{-f} (used for |
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@var{filename1}) and/or @var{-F} (used for @var{filename2}) option. |
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|
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By default, images with different size are considered identical if the larger |
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image contains only unallocated and/or zeroed sectors in the area after the end |
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of the other image. In addition, if any sector is not allocated in one image |
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and contains only zero bytes in the second one, it is evaluated as equal. You |
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can use Strict mode by specifying the @var{-s} option. When compare runs in |
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Strict mode, it fails in case image size differs or a sector is allocated in |
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one image and is not allocated in the second one. |
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|
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By default, compare prints out a result message. This message displays |
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information that both images are same or the position of the first different |
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byte. In addition, result message can report different image size in case |
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Strict mode is used. |
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|
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Compare exits with @code{0} in case the images are equal and with @code{1} |
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in case the images differ. Other exit codes mean an error occurred during |
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execution and standard error output should contain an error message. |
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The following table sumarizes all exit codes of the compare subcommand: |
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|
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@table @option |
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|
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@item 0 |
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Images are identical |
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@item 1 |
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Images differ |
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@item 2 |
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Error on opening an image |
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@item 3 |
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Error on checking a sector allocation |
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@item 4 |
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Error on reading data |
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|
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@end table |
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|
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@item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_id_or_name}] [-l @var{snapshot_param}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename} |
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|
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Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated) |
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to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c} |
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option) or use any format specific options like encryption (@code{-o} option). |
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|
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Only the formats @code{qcow} and @code{qcow2} support compression. The |
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compression is read-only. It means that if a compressed sector is |
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rewritten, then it is rewritten as uncompressed data. |
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|
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Image conversion is also useful to get smaller image when using a |
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growable format such as @code{qcow} or @code{cow}: the empty sectors |
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are detected and suppressed from the destination image. |
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|
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@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) |
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that must contain only zeros for qemu-img to create a sparse image during |
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conversion. If @var{sparse_size} is 0, the source will not be scanned for |
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unallocated or zero sectors, and the destination image will always be |
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fully allocated. |
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|
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You can use the @var{backing_file} option to force the output image to be |
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created as a copy on write image of the specified base image; the |
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@var{backing_file} should have the same content as the input's base image, |
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however the path, image format, etc may differ. |
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|
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If the @code{-n} option is specified, the target volume creation will be |
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skipped. This is useful for formats such as @code{rbd} if the target |
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volume has already been created with site specific options that cannot |
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be supplied through qemu-img. |
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|
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@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename} |
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|
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Give information about the disk image @var{filename}. Use it in |
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particular to know the size reserved on disk which can be different |
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from the displayed size. If VM snapshots are stored in the disk image, |
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they are displayed too. The command can output in the format @var{ofmt} |
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which is either @code{human} or @code{json}. |
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|
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If a disk image has a backing file chain, information about each disk image in |
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the chain can be recursively enumerated by using the option @code{--backing-chain}. |
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|
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For instance, if you have an image chain like: |
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|
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@example |
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base.qcow2 <- snap1.qcow2 <- snap2.qcow2 |
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@end example |
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|
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To enumerate information about each disk image in the above chain, starting from top to base, do: |
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|
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@example |
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qemu-img info --backing-chain snap2.qcow2 |
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@end example |
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|
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@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename} |
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|
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Dump the metadata of image @var{filename} and its backing file chain. |
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In particular, this commands dumps the allocation state of every sector |
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of @var{filename}, together with the topmost file that allocates it in |
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the backing file chain. |
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|
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Two option formats are possible. The default format (@code{human}) |
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only dumps known-nonzero areas of the file. Known-zero parts of the |
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file are omitted altogether, and likewise for parts that are not allocated |
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throughout the chain. @command{qemu-img} output will identify a file |
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from where the data can be read, and the offset in the file. Each line |
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will include four fields, the first three of which are hexadecimal |
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numbers. For example the first line of: |
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@example |
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Offset Length Mapped to File |
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0 0x20000 0x50000 /tmp/overlay.qcow2 |
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0x100000 0x10000 0x95380000 /tmp/backing.qcow2 |
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@end example |
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@noindent |
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means that 0x20000 (131072) bytes starting at offset 0 in the image are |
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available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting |
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at offset 0x50000 (327680). Data that is compressed, encrypted, or |
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otherwise not available in raw format will cause an error if @code{human} |
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format is in use. Note that file names can include newlines, thus it is |
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not safe to parse this output format in scripts. |
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|
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The alternative format @code{json} will return an array of dictionaries |
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in JSON format. It will include similar information in |
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the @code{start}, @code{length}, @code{offset} fields; |
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it will also include other more specific information: |
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@itemize @minus |
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@item |
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whether the sectors contain actual data or not (boolean field @code{data}; |
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if false, the sectors are either unallocated or stored as optimized |
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all-zero clusters); |
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|
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@item |
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whether the data is known to read as zero (boolean field @code{zero}); |
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|
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@item |
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in order to make the output shorter, the target file is expressed as |
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a @code{depth}; for example, a depth of 2 refers to the backing file |
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of the backing file of @var{filename}. |
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@end itemize |
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|
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In JSON format, the @code{offset} field is optional; it is absent in |
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cases where @code{human} format would omit the entry or exit with an error. |
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If @code{data} is false and the @code{offset} field is present, the |
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corresponding sectors in the file are not yet in use, but they are |
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preallocated. |
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|
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For more information, consult @file{include/block/block.h} in QEMU's |
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source code. |
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|
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@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename} |
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|
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List, apply, create or delete snapshots in image @var{filename}. |
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|
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@item rebase [-f @var{fmt}] [-t @var{cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename} |
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|
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Changes the backing file of an image. Only the formats @code{qcow2} and |
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@code{qed} support changing the backing file. |
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|
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The backing file is changed to @var{backing_file} and (if the image format of |
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@var{filename} supports this) the backing file format is changed to |
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@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty |
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string), then the image is rebased onto no backing file (i.e. it will exist |
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independently of any backing file). |
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|
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There are two different modes in which @code{rebase} can operate: |
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@table @option |
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@item Safe mode |
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This is the default mode and performs a real rebase operation. The new backing |
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file may differ from the old one and qemu-img rebase will take care of keeping |
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the guest-visible content of @var{filename} unchanged. |
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|
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In order to achieve this, any clusters that differ between @var{backing_file} |
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and the old backing file of @var{filename} are merged into @var{filename} |
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before actually changing the backing file. |
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|
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Note that the safe mode is an expensive operation, comparable to converting |
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an image. It only works if the old backing file still exists. |
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|
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@item Unsafe mode |
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qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the |
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backing file name and format of @var{filename} is changed without any checks |
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on the file contents. The user must take care of specifying the correct new |
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backing file, or the guest-visible content of the image will be corrupted. |
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|
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This mode is useful for renaming or moving the backing file to somewhere else. |
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It can be used without an accessible old backing file, i.e. you can use it to |
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fix an image whose backing file has already been moved/renamed. |
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@end table |
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|
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You can use @code{rebase} to perform a ``diff'' operation on two |
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disk images. This can be useful when you have copied or cloned |
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a guest, and you want to get back to a thin image on top of a |
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template or base image. |
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|
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Say that @code{base.img} has been cloned as @code{modified.img} by |
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copying it, and that the @code{modified.img} guest has run so there |
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are now some changes compared to @code{base.img}. To construct a thin |
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image called @code{diff.qcow2} that contains just the differences, do: |
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|
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@example |
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qemu-img create -f qcow2 -b modified.img diff.qcow2 |
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qemu-img rebase -b base.img diff.qcow2 |
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@end example |
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|
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At this point, @code{modified.img} can be discarded, since |
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@code{base.img + diff.qcow2} contains the same information. |
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|
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@item resize @var{filename} [+ | -]@var{size} |
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|
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Change the disk image as if it had been created with @var{size}. |
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|
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Before using this command to shrink a disk image, you MUST use file system and |
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partitioning tools inside the VM to reduce allocated file systems and partition |
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sizes accordingly. Failure to do so will result in data loss! |
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|
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After using this command to grow a disk image, you must use file system and |
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partitioning tools inside the VM to actually begin using the new space on the |
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device. |
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|
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@item amend [-f @var{fmt}] -o @var{options} @var{filename} |
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|
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Amends the image format specific @var{options} for the image file |
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@var{filename}. Not all file formats support this operation. |
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@end table |
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@c man end |
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|
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@ignore |
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@c man begin NOTES |
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Supported image file formats: |
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|
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@table @option |
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@item raw |
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|
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Raw disk image format (default). This format has the advantage of |
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being simple and easily exportable to all other emulators. If your |
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file system supports @emph{holes} (for example in ext2 or ext3 on |
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Linux or NTFS on Windows), then only the written sectors will reserve |
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space. Use @code{qemu-img info} to know the real size used by the |
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image or @code{ls -ls} on Unix/Linux. |
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|
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@item qcow2 |
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QEMU image format, the most versatile format. Use it to have smaller |
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images (useful if your filesystem does not supports holes, for example |
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on Windows), optional AES encryption, zlib based compression and |
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support of multiple VM snapshots. |
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|
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Supported options: |
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@table @code |
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@item compat |
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Determines the qcow2 version to use. @code{compat=0.10} uses the |
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traditional image format that can be read by any QEMU since 0.10. |
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@code{compat=1.1} enables image format extensions that only QEMU 1.1 and |
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newer understand (this is the default). Amongst others, this includes zero |
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clusters, which allow efficient copy-on-read for sparse images. |
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|
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@item backing_file |
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File name of a base image (see @option{create} subcommand) |
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@item backing_fmt |
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Image format of the base image |
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@item encryption |
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If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC. |
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|
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The use of encryption in qcow and qcow2 images is considered to be flawed by |
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modern cryptography standards, suffering from a number of design problems: |
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|
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@itemize @minus |
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@item The AES-CBC cipher is used with predictable initialization vectors based |
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on the sector number. This makes it vulnerable to chosen plaintext attacks |
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which can reveal the existence of encrypted data. |
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@item The user passphrase is directly used as the encryption key. A poorly |
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chosen or short passphrase will compromise the security of the encryption. |
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@item In the event of the passphrase being compromised there is no way to |
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change the passphrase to protect data in any qcow images. The files must |
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be cloned, using a different encryption passphrase in the new file. The |
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original file must then be securely erased using a program like shred, |
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though even this is ineffective with many modern storage technologies. |
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@end itemize |
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|
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Use of qcow / qcow2 encryption is thus strongly discouraged. Users are |
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recommended to use an alternative encryption technology such as the |
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Linux dm-crypt / LUKS system. |
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|
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@item cluster_size |
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Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster |
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sizes can improve the image file size whereas larger cluster sizes generally |
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provide better performance. |
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|
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@item preallocation |
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Preallocation mode (allowed values: off, metadata). An image with preallocated |
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metadata is initially larger but can improve performance when the image needs |
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to grow. |
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|
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@item lazy_refcounts |
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If this option is set to @code{on}, reference count updates are postponed with |
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the goal of avoiding metadata I/O and improving performance. This is |
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particularly interesting with @option{cache=writethrough} which doesn't batch |
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metadata updates. The tradeoff is that after a host crash, the reference count |
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tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img |
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check -r all} is required, which may take some time. |
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|
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This option can only be enabled if @code{compat=1.1} is specified. |
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|
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@end table |
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|
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@item Other |
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QEMU also supports various other image file formats for compatibility with |
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older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, |
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qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}. |
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For a more detailed description of these formats, see the QEMU Emulation User |
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Documentation. |
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|
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The main purpose of the block drivers for these formats is image conversion. |
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For running VMs, it is recommended to convert the disk images to either raw or |
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qcow2 in order to achieve good performance. |
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@end table |
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|
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|
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@c man end |
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|
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@setfilename qemu-img |
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@settitle QEMU disk image utility |
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|
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@c man begin SEEALSO |
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The HTML documentation of QEMU for more precise information and Linux |
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user mode emulator invocation. |
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@c man end |
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|
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@c man begin AUTHOR |
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Fabrice Bellard |
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@c man end |
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|
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@end ignore |