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diff --git a/qemu-img.texi b/qemu-img.texi deleted file mode 100644 index 20136fcb94..0000000000 --- a/qemu-img.texi +++ /dev/null @@ -1,798 +0,0 @@ -@example -@c man begin SYNOPSIS -@command{qemu-img} [@var{standard} @var{options}] @var{command} [@var{command} @var{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 - -Standard options: -@table @option -@item -h, --help -Display this help and exit -@item -V, --version -Display version information and exit -@item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}] -@findex --trace -@include qemu-option-trace.texi -@end table - -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 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 snapshot_param -is param used for internal snapshot, format is -'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]' - -@end table - -@table @option - -@item --object @var{objectdef} -is a QEMU user creatable object definition. See the @code{qemu(1)} manual -page for a description of the object properties. The most common object -type is a @code{secret}, which is used to supply passwords and/or encryption -keys. - -@item --image-opts -Indicates that the source @var{filename} parameter is to be interpreted as a -full option string, not a plain filename. This parameter is mutually -exclusive with the @var{-f} parameter. - -@item --target-image-opts -Indicates that the @var{output_filename} parameter(s) are to be interpreted as -a full option string, not a plain filename. This parameter is mutually -exclusive with the @var{-O} parameters. It is currently required to also use -the @var{-n} parameter to skip image creation. This restriction may be relaxed -in a future release. - -@item --force-share (-U) -If specified, @code{qemu-img} will open the image in shared mode, allowing -other QEMU processes to open it in write mode. For example, this can be used to -get the image information (with 'info' subcommand) when the image is used by a -running guest. Note that this could produce inconsistent results because of -concurrent metadata changes, etc. This option is only allowed when opening -images in read-only mode. - -@item --backing-chain -will enumerate information about backing files in a disk image chain. Refer -below for further description. - -@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 (compare, convert and rebase commands only). -If the @var{-p} option is not used for a command that supports it, the -progress is reported when the process receives a @code{SIGUSR1} or -@code{SIGINFO} signal. - -@item -q -Quiet mode - do not print any output (except errors). There's no progress bar -in case both @var{-q} and @var{-p} options are used. - -@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. - -@item -T @var{src_cache} -specifies the cache mode that should be used with the source file(s). 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 - -Parameters to compare subcommand: - -@table @option - -@item -f -First image format -@item -F -Second image format -@item -s -Strict mode - fail on different image size or sector allocation -@end table - -Parameters to convert subcommand: - -@table @option - -@item -n -Skip the creation of the target volume -@item -m -Number of parallel coroutines for the convert process -@item -W -Allow out-of-order writes to the destination. This option improves performance, -but is only recommended for preallocated devices like host devices or other -raw block devices. -@item -C -Try to use copy offloading to move data from source image to target. This may -improve performance if the data is remote, such as with NFS or iSCSI backends, -but will not automatically sparsify zero sectors, and may result in a fully -allocated target image depending on the host support for getting allocation -information. -@item --salvage -Try to ignore I/O errors when reading. Unless in quiet mode (@code{-q}), errors -will still be printed. Areas that cannot be read from the source will be -treated as containing only zeroes. -@end table - -Parameters to dd subcommand: - -@table @option - -@item bs=@var{block_size} -defines the block size -@item count=@var{blocks} -sets the number of input blocks to copy -@item if=@var{input} -sets the input file -@item of=@var{output} -sets the output file -@item skip=@var{blocks} -sets the number of input blocks to skip -@end table - -Command description: - -@table @option - -@item amend [--object @var{objectdef}] [--image-opts] [-p] [-q] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename} - -Amends the image format specific @var{options} for the image file -@var{filename}. Not all file formats support this operation. - -@item bench [-c @var{count}] [-d @var{depth}] [-f @var{fmt}] [--flush-interval=@var{flush_interval}] [-n] [-i @var{aio}] [--no-drain] [-o @var{offset}] [--pattern=@var{pattern}] [-q] [-s @var{buffer_size}] [-S @var{step_size}] [-t @var{cache}] [-w] [-U] @var{filename} - -Run a simple sequential I/O benchmark on the specified image. If @code{-w} is -specified, a write test is performed, otherwise a read test is performed. - -A total number of @var{count} I/O requests is performed, each @var{buffer_size} -bytes in size, and with @var{depth} requests in parallel. The first request -starts at the position given by @var{offset}, each following request increases -the current position by @var{step_size}. If @var{step_size} is not given, -@var{buffer_size} is used for its value. - -If @var{flush_interval} is specified for a write test, the request queue is -drained and a flush is issued before new writes are made whenever the number of -remaining requests is a multiple of @var{flush_interval}. If additionally -@code{--no-drain} is specified, a flush is issued without draining the request -queue first. - -If @code{-n} is specified, the native AIO backend is used if possible. On -Linux, this option only works if @code{-t none} or @code{-t directsync} is -specified as well. - -If @code{-i} is specified, aio option can be used to specify different AIO -backends: @var{threads}, @var{native} or @var{io_uring}. - -For write tests, by default a buffer filled with zeros is written. This can be -overridden with a pattern byte specified by @var{pattern}. - -@item check [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] [-U] @var{filename} - -Perform a consistency check on the disk image @var{filename}. The command can -output in the format @var{ofmt} which is either @code{human} or @code{json}. -The JSON output is an object of QAPI type @code{ImageCheck}. - -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. - -In case the image does not have any inconsistencies, check exits with @code{0}. -Other exit codes indicate the kind of inconsistency found or if another error -occurred. The following table summarizes all exit codes of the check subcommand: - -@table @option - -@item 0 -Check completed, the image is (now) consistent -@item 1 -Check not completed because of internal errors -@item 2 -Check completed, image is corrupted -@item 3 -Check completed, image has leaked clusters, but is not corrupted -@item 63 -Checks are not supported by the image format - -@end table - -If @code{-r} is specified, exit codes representing the image state refer to the -state after (the attempt at) repairing it. That is, a successful @code{-r all} -will yield the exit code 0, independently of the image state before. - -@item commit [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename} - -Commit the changes recorded in @var{filename} in its base image or backing file. -If the backing file is smaller than the snapshot, then the backing file will be -resized to be the same size as the snapshot. If the snapshot is smaller than -the backing file, the backing file will not be truncated. If you want the -backing file to match the size of the smaller snapshot, you can safely truncate -it yourself once the commit operation successfully completes. - -The image @var{filename} is emptied after the operation has succeeded. If you do -not need @var{filename} afterwards and intend to drop it, you may skip emptying -@var{filename} by specifying the @code{-d} flag. - -If the backing chain of the given image file @var{filename} has more than one -layer, the backing file into which the changes will be committed may be -specified as @var{base} (which has to be part of @var{filename}'s backing -chain). If @var{base} is not specified, the immediate backing file of the top -image (which is @var{filename}) will be used. Note that after a commit operation -all images between @var{base} and the top image will be invalid and may return -garbage data when read. For this reason, @code{-b} implies @code{-d} (so that -the top image stays valid). - -@item compare [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-q] [-s] [-U] @var{filename1} @var{filename2} - -Check if two images have the same content. You can compare images with -different format or settings. - -The format is probed unless you specify it by @var{-f} (used for -@var{filename1}) and/or @var{-F} (used for @var{filename2}) option. - -By default, images with different size are considered identical if the larger -image contains only unallocated and/or zeroed sectors in the area after the end -of the other image. In addition, if any sector is not allocated in one image -and contains only zero bytes in the second one, it is evaluated as equal. You -can use Strict mode by specifying the @var{-s} option. When compare runs in -Strict mode, it fails in case image size differs or a sector is allocated in -one image and is not allocated in the second one. - -By default, compare prints out a result message. This message displays -information that both images are same or the position of the first different -byte. In addition, result message can report different image size in case -Strict mode is used. - -Compare exits with @code{0} in case the images are equal and with @code{1} -in case the images differ. Other exit codes mean an error occurred during -execution and standard error output should contain an error message. -The following table sumarizes all exit codes of the compare subcommand: - -@table @option - -@item 0 -Images are identical -@item 1 -Images differ -@item 2 -Error on opening an image -@item 3 -Error on checking a sector allocation -@item 4 -Error on reading data - -@end table - -@item convert [--object @var{objectdef}] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-B @var{backing_file}] [-o @var{options}] [-l @var{snapshot_param}] [-S @var{sparse_size}] [-m @var{num_coroutines}] [-W] @var{filename} [@var{filename2} [...]] @var{output_filename} - -Convert the disk image @var{filename} or a snapshot @var{snapshot_param} -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}: the empty sectors are detected and -suppressed from the destination image. - -@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) -that must contain only zeros for qemu-img to create a sparse image during -conversion. If @var{sparse_size} is 0, the source will not be scanned for -unallocated or zero sectors, and the destination image will always be -fully allocated. - -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. - -If a relative path name is given, the backing file is looked up relative to -the directory containing @var{output_filename}. - -If the @code{-n} option is specified, the target volume creation will be -skipped. This is useful for formats such as @code{rbd} if the target -volume has already been created with site specific options that cannot -be supplied through qemu-img. - -Out of order writes can be enabled with @code{-W} to improve performance. -This is only recommended for preallocated devices like host devices or other -raw block devices. Out of order write does not work in combination with -creating compressed images. - -@var{num_coroutines} specifies how many coroutines work in parallel during -the convert process (defaults to 8). - -@item create [--object @var{objectdef}] [-q] [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-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). - -If a relative path name is given, the backing file is looked up relative to -the directory containing @var{filename}. - -Note that a given backing file will be opened to check that it is valid. Use -the @code{-u} option to enable unsafe backing file mode, which means that the -image will be created even if the associated backing file cannot be opened. A -matching backing file must be created or additional options be used to make the -backing file specification valid when you want to use an image created this -way. - -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 dd [--image-opts] [-U] [-f @var{fmt}] [-O @var{output_fmt}] [bs=@var{block_size}] [count=@var{blocks}] [skip=@var{blocks}] if=@var{input} of=@var{output} - -Dd copies from @var{input} file to @var{output} file converting it from -@var{fmt} format to @var{output_fmt} format. - -The data is by default read and written using blocks of 512 bytes but can be -modified by specifying @var{block_size}. If count=@var{blocks} is specified -dd will stop reading input after reading @var{blocks} input blocks. - -The size syntax is similar to dd(1)'s size syntax. - -@item info [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] [-U] @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. - -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 - -The command can output in the format @var{ofmt} which is either @code{human} or -@code{json}. The JSON output is an object of QAPI type @code{ImageInfo}; with -@code{--backing-chain}, it is an array of @code{ImageInfo} objects. - -@code{--output=human} reports the following information (for every image in the -chain): -@table @var -@item image -The image file name - -@item file format -The image format - -@item virtual size -The size of the guest disk - -@item disk size -How much space the image file occupies on the host file system (may be shown as -0 if this information is unavailable, e.g. because there is no file system) - -@item cluster_size -Cluster size of the image format, if applicable - -@item encrypted -Whether the image is encrypted (only present if so) - -@item cleanly shut down -This is shown as @code{no} if the image is dirty and will have to be -auto-repaired the next time it is opened in qemu. - -@item backing file -The backing file name, if present - -@item backing file format -The format of the backing file, if the image enforces it - -@item Snapshot list -A list of all internal snapshots - -@item Format specific information -Further information whose structure depends on the image format. This section -is a textual representation of the respective @code{ImageInfoSpecific*} QAPI -object (e.g. @code{ImageInfoSpecificQCow2} for qcow2 images). -@end table - -@item map [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--output=@var{ofmt}] [-U] @var{filename} - -Dump the metadata of image @var{filename} and its backing file chain. -In particular, this commands dumps the allocation state of every sector -of @var{filename}, together with the topmost file that allocates it in -the backing file chain. - -Two option formats are possible. The default format (@code{human}) -only dumps known-nonzero areas of the file. Known-zero parts of the -file are omitted altogether, and likewise for parts that are not allocated -throughout the chain. @command{qemu-img} output will identify a file -from where the data can be read, and the offset in the file. Each line -will include four fields, the first three of which are hexadecimal -numbers. For example the first line of: -@example -Offset Length Mapped to File -0 0x20000 0x50000 /tmp/overlay.qcow2 -0x100000 0x10000 0x95380000 /tmp/backing.qcow2 -@end example -@noindent -means that 0x20000 (131072) bytes starting at offset 0 in the image are -available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting -at offset 0x50000 (327680). Data that is compressed, encrypted, or -otherwise not available in raw format will cause an error if @code{human} -format is in use. Note that file names can include newlines, thus it is -not safe to parse this output format in scripts. - -The alternative format @code{json} will return an array of dictionaries -in JSON format. It will include similar information in -the @code{start}, @code{length}, @code{offset} fields; -it will also include other more specific information: -@itemize @minus -@item -whether the sectors contain actual data or not (boolean field @code{data}; -if false, the sectors are either unallocated or stored as optimized -all-zero clusters); - -@item -whether the data is known to read as zero (boolean field @code{zero}); - -@item -in order to make the output shorter, the target file is expressed as -a @code{depth}; for example, a depth of 2 refers to the backing file -of the backing file of @var{filename}. -@end itemize - -In JSON format, the @code{offset} field is optional; it is absent in -cases where @code{human} format would omit the entry or exit with an error. -If @code{data} is false and the @code{offset} field is present, the -corresponding sectors in the file are not yet in use, but they are -preallocated. - -For more information, consult @file{include/block/block.h} in QEMU's -source code. - -@item measure [--output=@var{ofmt}] [-O @var{output_fmt}] [-o @var{options}] [--size @var{N} | [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-l @var{snapshot_param}] @var{filename}] - -Calculate the file size required for a new image. This information can be used -to size logical volumes or SAN LUNs appropriately for the image that will be -placed in them. The values reported are guaranteed to be large enough to fit -the image. The command can output in the format @var{ofmt} which is either -@code{human} or @code{json}. The JSON output is an object of QAPI type -@code{BlockMeasureInfo}. - -If the size @var{N} is given then act as if creating a new empty image file -using @command{qemu-img create}. If @var{filename} is given then act as if -converting an existing image file using @command{qemu-img convert}. The format -of the new file is given by @var{output_fmt} while the format of an existing -file is given by @var{fmt}. - -A snapshot in an existing image can be specified using @var{snapshot_param}. - -The following fields are reported: -@example -required size: 524288 -fully allocated size: 1074069504 -@end example - -The @code{required size} is the file size of the new image. It may be smaller -than the virtual disk size if the image format supports compact representation. - -The @code{fully allocated size} is the file size of the new image once data has -been written to all sectors. This is the maximum size that the image file can -occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, -and other advanced image format features. - -@item snapshot [--object @var{objectdef}] [--image-opts] [-U] [-q] [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot}] @var{filename} - -List, apply, create or delete snapshots in image @var{filename}. - -@item rebase [--object @var{objectdef}] [--image-opts] [-U] [-q] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_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). - -If a relative path name is given, the backing file is looked up relative to -the directory containing @var{filename}. - -@var{cache} specifies the cache mode to be used for @var{filename}, whereas -@var{src_cache} specifies the cache mode for reading backing files. - -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 [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--preallocation=@var{prealloc}] [-q] [--shrink] @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! - -When shrinking images, the @code{--shrink} option must be given. This informs -qemu-img that the user acknowledges all loss of data beyond the truncated -image's end. - -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. - -When growing an image, the @code{--preallocation} option may be used to specify -how the additional image area should be allocated on the host. See the format -description in the @code{NOTES} section which values are allowed. Using this -option may result in slightly more data being allocated than necessary. - -@end table -@c man end - -@ignore -@c man begin NOTES -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. - -Supported options: -@table @code -@item preallocation -Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}). -@code{falloc} mode preallocates space for image by calling posix_fallocate(). -@code{full} mode preallocates space for image by writing data to underlying -storage. This data may or may not be zero, depending on the storage location. -@end table - -@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 compat -Determines the qcow2 version to use. @code{compat=0.10} uses the -traditional image format that can be read by any QEMU since 0.10. -@code{compat=1.1} enables image format extensions that only QEMU 1.1 and -newer understand (this is the default). Amongst others, this includes zero -clusters, which allow efficient copy-on-read for sparse images. - -@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 with 128-bit AES-CBC. - -The use of encryption in qcow and qcow2 images is considered to be flawed by -modern cryptography standards, suffering from a number of design problems: - -@itemize @minus -@item -The AES-CBC cipher is used with predictable initialization vectors based -on the sector number. This makes it vulnerable to chosen plaintext attacks -which can reveal the existence of encrypted data. -@item -The user passphrase is directly used as the encryption key. A poorly -chosen or short passphrase will compromise the security of the encryption. -@item -In the event of the passphrase being compromised there is no way to -change the passphrase to protect data in any qcow images. The files must -be cloned, using a different encryption passphrase in the new file. The -original file must then be securely erased using a program like shred, -though even this is ineffective with many modern storage technologies. -@item -Initialization vectors used to encrypt sectors are based on the -guest virtual sector number, instead of the host physical sector. When -a disk image has multiple internal snapshots this means that data in -multiple physical sectors is encrypted with the same initialization -vector. With the CBC mode, this opens the possibility of watermarking -attacks if the attack can collect multiple sectors encrypted with the -same IV and some predictable data. Having multiple qcow2 images with -the same passphrase also exposes this weakness since the passphrase -is directly used as the key. -@end itemize - -Use of qcow / qcow2 encryption is thus strongly discouraged. Users are -recommended to use an alternative encryption technology such as the -Linux dm-crypt / LUKS system. - -@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: @code{off}, @code{metadata}, @code{falloc}, -@code{full}). An image with preallocated metadata is initially larger but can -improve performance when the image needs to grow. @code{falloc} and @code{full} -preallocations are like the same options of @code{raw} format, but sets up -metadata also. - -@item lazy_refcounts -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. - -@item nocow -If this option is set to @code{on}, it will turn off COW of the file. It's only -valid on btrfs, no effect on other file systems. - -Btrfs has low performance when hosting a VM image file, even more when the guest -on the VM also using btrfs as file system. Turning off COW is a way to mitigate -this bad performance. Generally there are two ways to turn off COW on btrfs: -a) Disable it by mounting with nodatacow, then all newly created files will be -NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option -does. - -Note: this option is only valid to new or empty files. If there is an existing -file which is COW and has data blocks already, it couldn't be changed to NOCOW -by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if -the NOCOW flag is set or not (Capital 'C' is NOCOW flag). - -@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 |