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diff --git a/qemu/qemu-img.texi b/qemu/qemu-img.texi new file mode 100644 index 000000000..0a1ab3598 --- /dev/null +++ b/qemu/qemu-img.texi @@ -0,0 +1,554 @@ +@example +@c man begin SYNOPSIS +usage: qemu-img command [command options] +@c man end +@end example + +@c man begin DESCRIPTION +qemu-img allows you to create, convert and modify images offline. It can handle +all image formats supported by QEMU. + +@b{Warning:} Never use qemu-img to modify images in use by a running virtual +machine or any other process; this may destroy the image. Also, be aware that +querying an image that is being modified by another process may encounter +inconsistent state. +@c man end + +@c man begin OPTIONS + +The following commands are supported: + +@include qemu-img-cmds.texi + +Command parameters: +@table @var +@item filename + is a disk image filename +@item fmt +is the disk image format. It is guessed automatically in most cases. See below +for a description of the supported disk formats. + +@item --backing-chain +will enumerate information about backing files in a disk image chain. Refer +below for further description. + +@item size +is the disk image size in bytes. Optional suffixes @code{k} or @code{K} +(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M) +and T (terabyte, 1024G) are supported. @code{b} is ignored. + +@item output_filename +is the destination disk image filename + +@item output_fmt + is the destination format +@item options +is a comma separated list of format specific options in a +name=value format. Use @code{-o ?} for an overview of the options supported +by the used format or see the format descriptions below for details. +@item snapshot_param +is param used for internal snapshot, format is +'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]' +@item snapshot_id_or_name +is deprecated, use snapshot_param instead + +@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} 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 on different image size or sector allocation +@end table + +Parameters to convert subcommand: + +@table @option + +@item -n +Skip the creation of the target volume +@end table + +Command description: + +@table @option +@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @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}. + +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 create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}] + +Create the new disk image @var{filename} of size @var{size} and format +@var{fmt}. Depending on the file format, you can add one or more @var{options} +that enable additional features of this format. + +If the option @var{backing_file} is specified, then the image will record +only the differences from @var{backing_file}. No size needs to be specified in +this case. @var{backing_file} will never be modified unless you use the +@code{commit} monitor command (or qemu-img commit). + +The size can also be specified using the @var{size} option with @code{-o}, +it doesn't need to be specified separately in this case. + +@item commit [-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. For reasons of consistency, +explicitly specifying @var{base} will always imply @code{-d} (since emptying an +image after committing to an indirect backing file would lead to different data +being read from the image due to content in the intermediate backing chain +overruling the commit target). + +@item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @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 [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_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} + +Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated) +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 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. + +@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename} + +Give information about the disk image @var{filename}. Use it in +particular to know the size reserved on disk which can be different +from the displayed size. If VM snapshots are stored in the disk image, +they are displayed too. The command can output in the format @var{ofmt} +which is either @code{human} or @code{json}. + +If a disk image has a backing file chain, information about each disk image in +the chain can be recursively enumerated by using the option @code{--backing-chain}. + +For instance, if you have an image chain like: + +@example +base.qcow2 <- snap1.qcow2 <- snap2.qcow2 +@end example + +To enumerate information about each disk image in the above chain, starting from top to base, do: + +@example +qemu-img info --backing-chain snap2.qcow2 +@end example + +@item map [-f @var{fmt}] [--output=@var{ofmt}] @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 snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename} + +List, apply, create or delete snapshots in image @var{filename}. + +@item rebase [-f @var{fmt}] [-t @var{cache}] [-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). + +@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 @var{filename} [+ | -]@var{size} + +Change the disk image as if it had been created with @var{size}. + +Before using this command to shrink a disk image, you MUST use file system and +partitioning tools inside the VM to reduce allocated file systems and partition +sizes accordingly. Failure to do so will result in data loss! + +After using this command to grow a disk image, you must use file system and +partitioning tools inside the VM to actually begin using the new space on the +device. + +@item amend [-p] [-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. +@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 zeros to underlying +storage. +@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. +@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 |