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![Daniel P. Berrange](/assets/img/avatar_default.png)
The qemu-img.texi / qemu-doc.texi files currently describe the qcow2/qcow2 encryption thus "Encryption uses the AES format which is very secure (128 bit keys). Use a long password (16 characters) to get maximum protection." While AES is indeed a strong encryption system, the way that QCow/QCow2 use it results in a poor/weak encryption system. Due to the use of predictable IVs, based on the sector number extended to 128 bits, it is vulnerable to chosen plaintext attacks which can reveal the existence of encrypted data. The direct use of the user passphrase as the encryption key also leads to an inability to change the passphrase of an image. If passphrase is ever compromised the image data will all be vulnerable, since it cannot be re-encrypted. The admin has to clone the image files with a new passphrase and then use a program like shred to secure erase all the old files. Recommend against any use of QCow/QCow2 encryption, directing users to dm-crypt / LUKS which can meet modern cryptography best practices. [Changed "Qcow" to "qcow" for consistency. --Stefan] Signed-off-by: Daniel P. Berrange <berrange@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
484 lines
19 KiB
Plaintext
484 lines
19 KiB
Plaintext
@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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@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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@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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@c man begin OPTIONS
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The following commands are supported:
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@include qemu-img-cmds.texi
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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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@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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@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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@item output_filename
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is the destination disk image filename
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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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@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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Parameters to snapshot subcommand:
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@table @option
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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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Parameters to compare subcommand:
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@table @option
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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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Parameters to convert subcommand:
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@table @option
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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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Command description:
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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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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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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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Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
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consistency checks.
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@item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]
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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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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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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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@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}
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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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@item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2}
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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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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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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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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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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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@table @option
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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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@end table
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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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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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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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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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@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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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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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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@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
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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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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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For instance, if you have an image chain like:
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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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To enumerate information about each disk image in the above chain, starting from top to base, do:
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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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@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
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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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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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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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@item
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whether the data is known to read as zero (boolean field @code{zero});
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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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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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For more information, consult @file{include/block/block.h} in QEMU's
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source code.
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@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}
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List, apply, create or delete snapshots in image @var{filename}.
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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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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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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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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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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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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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@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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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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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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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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@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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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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@item resize @var{filename} [+ | -]@var{size}
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Change the disk image as if it had been created with @var{size}.
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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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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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@item amend [-f @var{fmt}] -o @var{options} @var{filename}
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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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@ignore
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@c man begin NOTES
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Supported image file formats:
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@table @option
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@item raw
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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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@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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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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@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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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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@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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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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@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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@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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@item lazy_refcounts
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If this option is set to @code{on}, reference count updates are postponed with
|
|
the goal of avoiding metadata I/O and improving performance. This is
|
|
particularly interesting with @option{cache=writethrough} which doesn't batch
|
|
metadata updates. The tradeoff is that after a host crash, the reference count
|
|
tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
|
|
check -r all} is required, which may take some time.
|
|
|
|
This option can only be enabled if @code{compat=1.1} is specified.
|
|
|
|
@end table
|
|
|
|
@item Other
|
|
QEMU also supports various other image file formats for compatibility with
|
|
older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
|
|
qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
|
|
For a more detailed description of these formats, see the QEMU Emulation User
|
|
Documentation.
|
|
|
|
The main purpose of the block drivers for these formats is image conversion.
|
|
For running VMs, it is recommended to convert the disk images to either raw or
|
|
qcow2 in order to achieve good performance.
|
|
@end table
|
|
|
|
|
|
@c man end
|
|
|
|
@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
|