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diff --git a/docs/interop/qemu-img.rst b/docs/interop/qemu-img.rst deleted file mode 100644 index 0080f83a76..0000000000 --- a/docs/interop/qemu-img.rst +++ /dev/null @@ -1,832 +0,0 @@ -QEMU disk image utility -======================= - -Synopsis --------- - -**qemu-img** [*standard options*] *command* [*command options*] - -Description ------------ - -qemu-img allows you to create, convert and modify images offline. It can handle -all image formats supported by QEMU. - -**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. - -Options -------- - -.. program:: qemu-img - -Standard options: - -.. option:: -h, --help - - Display this help and exit - -.. option:: -V, --version - - Display version information and exit - -.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE] - - .. include:: ../qemu-option-trace.rst.inc - -The following commands are supported: - -.. hxtool-doc:: qemu-img-cmds.hx - -Command parameters: - -*FILENAME* is a disk image filename. - -*FMT* is the disk image format. It is guessed automatically in most -cases. See below for a description of the supported disk formats. - -*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or -``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte, -1024M) and T (terabyte, 1024G) are supported. ``b`` is ignored. - -*OUTPUT_FILENAME* is the destination disk image filename. - -*OUTPUT_FMT* is the destination format. - -*OPTIONS* is a comma separated list of format specific options in a -name=value format. Use ``-o ?`` for an overview of the options supported -by the used format or see the format descriptions below for details. - -*SNAPSHOT_PARAM* is param used for internal snapshot, format is -'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'. - -.. - Note the use of a new 'program'; otherwise Sphinx complains about - the -h option appearing both in the above option list and this one. - -.. program:: qemu-img-common-opts - -.. option:: --object OBJECTDEF - - is a QEMU user creatable object definition. See the :manpage:`qemu(1)` - manual page for a description of the object properties. The most common - object type is a ``secret``, which is used to supply passwords and/or - encryption keys. - -.. option:: --image-opts - - Indicates that the source *FILENAME* parameter is to be interpreted as a - full option string, not a plain filename. This parameter is mutually - exclusive with the *-f* parameter. - -.. option:: --target-image-opts - - Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as - a full option string, not a plain filename. This parameter is mutually - exclusive with the *-O* parameters. It is currently required to also use - the *-n* parameter to skip image creation. This restriction may be relaxed - in a future release. - -.. option:: --force-share (-U) - - If specified, ``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. - -.. option:: --backing-chain - - Will enumerate information about backing files in a disk image chain. Refer - below for further description. - -.. option:: -c - - Indicates that target image must be compressed (qcow format only). - -.. option:: -h - - With or without a command, shows help and lists the supported formats. - -.. option:: -p - - Display progress bar (compare, convert and rebase commands only). - If the *-p* option is not used for a command that supports it, the - progress is reported when the process receives a ``SIGUSR1`` or - ``SIGINFO`` signal. - -.. option:: -q - - Quiet mode - do not print any output (except errors). There's no progress bar - in case both *-q* and *-p* options are used. - -.. option:: -S 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 - ``k`` for kilobytes. - -.. option:: -t CACHE - - Specifies the cache mode that should be used with the (destination) file. See - the documentation of the emulator's ``-drive cache=...`` option for allowed - values. - -.. option:: -T SRC_CACHE - - Specifies the cache mode that should be used with the source file(s). See - the documentation of the emulator's ``-drive cache=...`` option for allowed - values. - -Parameters to snapshot subcommand: - -.. program:: qemu-img-snapshot - -.. option:: snapshot - - Is the name of the snapshot to create, apply or delete - -.. option:: -a - - Applies a snapshot (revert disk to saved state) - -.. option:: -c - - Creates a snapshot - -.. option:: -d - - Deletes a snapshot - -.. option:: -l - - Lists all snapshots in the given image - -Parameters to compare subcommand: - -.. program:: qemu-img-compare - -.. option:: -f - - First image format - -.. option:: -F - - Second image format - -.. option:: -s - - Strict mode - fail on different image size or sector allocation - -Parameters to convert subcommand: - -.. program:: qemu-img-convert - -.. option:: -n - - Skip the creation of the target volume - -.. option:: -m - - Number of parallel coroutines for the convert process - -.. option:: -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. - -.. option:: -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. - -.. option:: --salvage - - Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors - will still be printed. Areas that cannot be read from the source will be - treated as containing only zeroes. - -.. option:: --target-is-zero - - Assume that reading the destination image will always return - zeros. This parameter is mutually exclusive with a destination image - that has a backing file. It is required to also use the ``-n`` - parameter to skip image creation. - -Parameters to dd subcommand: - -.. program:: qemu-img-dd - -.. option:: bs=BLOCK_SIZE - - Defines the block size - -.. option:: count=BLOCKS - - Sets the number of input blocks to copy - -.. option:: if=INPUT - - Sets the input file - -.. option:: of=OUTPUT - - Sets the output file - -.. option:: skip=BLOCKS - - Sets the number of input blocks to skip - -Command description: - -.. program:: qemu-img-commands - -.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] -o OPTIONS FILENAME - - Amends the image format specific *OPTIONS* for the image file - *FILENAME*. Not all file formats support this operation. - -.. option:: bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME - - Run a simple sequential I/O benchmark on the specified image. If ``-w`` is - specified, a write test is performed, otherwise a read test is performed. - - A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE* - bytes in size, and with *DEPTH* requests in parallel. The first request - starts at the position given by *OFFSET*, each following request increases - the current position by *STEP_SIZE*. If *STEP_SIZE* is not given, - *BUFFER_SIZE* is used for its value. - - If *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 *FLUSH_INTERVAL*. If additionally - ``--no-drain`` is specified, a flush is issued without draining the request - queue first. - - if ``-i`` is specified, *AIO* option can be used to specify different - AIO backends: ``threads``, ``native`` or ``io_uring``. - - If ``-n`` is specified, the native AIO backend is used if possible. On - Linux, this option only works if ``-t none`` or ``-t directsync`` is - specified as well. - - For write tests, by default a buffer filled with zeros is written. This can be - overridden with a pattern byte specified by *PATTERN*. - -.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME - - Perform a consistency check on the disk image *FILENAME*. The command can - output in the format *OFMT* which is either ``human`` or ``json``. - The JSON output is an object of QAPI type ``ImageCheck``. - - If ``-r`` is specified, qemu-img tries to repair any inconsistencies found - during the check. ``-r leaks`` repairs only cluster leaks, whereas - ``-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 ``qcow2``, ``qed`` and ``vdi`` support - consistency checks. - - In case the image does not have any inconsistencies, check exits with ``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: - - 0 - Check completed, the image is (now) consistent - 1 - Check not completed because of internal errors - 2 - Check completed, image is corrupted - 3 - Check completed, image has leaked clusters, but is not corrupted - 63 - Checks are not supported by the image format - - If ``-r`` is specified, exit codes representing the image state refer to the - state after (the attempt at) repairing it. That is, a successful ``-r all`` - will yield the exit code 0, independently of the image state before. - -.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME - - Commit the changes recorded in *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 *FILENAME* is emptied after the operation has succeeded. If you do - not need *FILENAME* afterwards and intend to drop it, you may skip emptying - *FILENAME* by specifying the ``-d`` flag. - - If the backing chain of the given image file *FILENAME* has more than one - layer, the backing file into which the changes will be committed may be - specified as *BASE* (which has to be part of *FILENAME*'s backing - chain). If *BASE* is not specified, the immediate backing file of the top - image (which is *FILENAME*) will be used. Note that after a commit operation - all images between *BASE* and the top image will be invalid and may return - garbage data when read. For this reason, ``-b`` implies ``-d`` (so that - the top image stays valid). - -.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 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 ``-f`` (used for - *FILENAME1*) and/or ``-F`` (used for *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 ``-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 ``0`` in case the images are equal and with ``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: - - 0 - Images are identical - 1 - Images differ - 2 - Error on opening an image - 3 - Error on checking a sector allocation - 4 - Error on reading data - -.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME - - Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* - to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can - be optionally compressed (``-c`` option) or use any format specific - options like encryption (``-o`` option). - - Only the formats ``qcow`` and ``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 ``qcow``: the empty sectors are detected and - suppressed from the destination image. - - *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 *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 *BACKING_FILE* option to force the output image to be - created as a copy on write image of the specified base image; the - *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 *OUTPUT_FILENAME*. - - If the ``-n`` option is specified, the target volume creation will be - skipped. This is useful for formats such as ``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 ``-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. - - *NUM_COROUTINES* specifies how many coroutines work in parallel during - the convert process (defaults to 8). - -.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE] - - Create the new disk image *FILENAME* of size *SIZE* and format - *FMT*. Depending on the file format, you can add one or more *OPTIONS* - that enable additional features of this format. - - If the option *BACKING_FILE* is specified, then the image will record - only the differences from *BACKING_FILE*. No size needs to be specified in - this case. *BACKING_FILE* will never be modified unless you use the - ``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 *FILENAME*. - - Note that a given backing file will be opened to check that it is valid. Use - the ``-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 *SIZE* option with ``-o``, - it doesn't need to be specified separately in this case. - - -.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT - - dd copies from *INPUT* file to *OUTPUT* file converting it from - *FMT* format to *OUTPUT_FMT* format. - - The data is by default read and written using blocks of 512 bytes but can be - modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified - dd will stop reading input after reading *BLOCKS* input blocks. - - The size syntax is similar to :manpage:`dd(1)`'s size syntax. - -.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME - - Give information about the disk image *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 ``--backing-chain``. - - For instance, if you have an image chain like: - - :: - - base.qcow2 <- snap1.qcow2 <- snap2.qcow2 - - To enumerate information about each disk image in the above chain, starting from top to base, do: - - :: - - qemu-img info --backing-chain snap2.qcow2 - - The command can output in the format *OFMT* which is either ``human`` or - ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with - ``--backing-chain``, it is an array of ``ImageInfo`` objects. - - ``--output=human`` reports the following information (for every image in the - chain): - - *image* - The image file name - - *file format* - The image format - - *virtual size* - The size of the guest disk - - *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) - - *cluster_size* - Cluster size of the image format, if applicable - - *encrypted* - Whether the image is encrypted (only present if so) - - *cleanly shut down* - This is shown as ``no`` if the image is dirty and will have to be - auto-repaired the next time it is opened in qemu. - - *backing file* - The backing file name, if present - - *backing file format* - The format of the backing file, if the image enforces it - - *Snapshot list* - A list of all internal snapshots - - *Format specific information* - Further information whose structure depends on the image format. This - section is a textual representation of the respective - ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` - for qcow2 images). - -.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [-U] FILENAME - - Dump the metadata of image *FILENAME* and its backing file chain. - In particular, this commands dumps the allocation state of every sector - of *FILENAME*, together with the topmost file that allocates it in - the backing file chain. - - Two option formats are possible. The default format (``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. ``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: - - :: - - Offset Length Mapped to File - 0 0x20000 0x50000 /tmp/overlay.qcow2 - 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 - - means that 0x20000 (131072) bytes starting at offset 0 in the image are - available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting - at offset 0x50000 (327680). Data that is compressed, encrypted, or - otherwise not available in raw format will cause an error if ``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 ``json`` will return an array of dictionaries - in JSON format. It will include similar information in - the ``start``, ``length``, ``offset`` fields; - it will also include other more specific information: - - - whether the sectors contain actual data or not (boolean field ``data``; - if false, the sectors are either unallocated or stored as optimized - all-zero clusters); - - whether the data is known to read as zero (boolean field ``zero``); - - in order to make the output shorter, the target file is expressed as - a ``depth``; for example, a depth of 2 refers to the backing file - of the backing file of *FILENAME*. - - In JSON format, the ``offset`` field is optional; it is absent in - cases where ``human`` format would omit the entry or exit with an error. - If ``data`` is false and the ``offset`` field is present, the - corresponding sectors in the file are not yet in use, but they are - preallocated. - - For more information, consult ``include/block/block.h`` in QEMU's - source code. - -.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] 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 *OFMT* which is either ``human`` or ``json``. - The JSON output is an object of QAPI type ``BlockMeasureInfo``. - - If the size *N* is given then act as if creating a new empty image file - using ``qemu-img create``. If *FILENAME* is given then act as if - converting an existing image file using ``qemu-img convert``. The format - of the new file is given by *OUTPUT_FMT* while the format of an existing - file is given by *FMT*. - - A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. - - The following fields are reported: - - :: - - required size: 524288 - fully allocated size: 1074069504 - - The ``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 ``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. - -.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME - - List, apply, create or delete snapshots in image *FILENAME*. - -.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME - - Changes the backing file of an image. Only the formats ``qcow2`` and - ``qed`` support changing the backing file. - - The backing file is changed to *BACKING_FILE* and (if the image format of - *FILENAME* supports this) the backing file format is changed to - *BACKING_FMT*. If *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 *FILENAME*. - - *CACHE* specifies the cache mode to be used for *FILENAME*, whereas - *SRC_CACHE* specifies the cache mode for reading backing files. - - There are two different modes in which ``rebase`` can operate: - - 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 *FILENAME* - unchanged. - - In order to achieve this, any clusters that differ between - *BACKING_FILE* and the old backing file of *FILENAME* are merged - into *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. - - Unsafe mode - qemu-img uses the unsafe mode if ``-u`` is specified. In this - mode, only the backing file name and format of *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. - - You can use ``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 ``base.img`` has been cloned as ``modified.img`` by - copying it, and that the ``modified.img`` guest has run so there - are now some changes compared to ``base.img``. To construct a thin - image called ``diff.qcow2`` that contains just the differences, do: - - :: - - qemu-img create -f qcow2 -b modified.img diff.qcow2 - qemu-img rebase -b base.img diff.qcow2 - - At this point, ``modified.img`` can be discarded, since - ``base.img + diff.qcow2`` contains the same information. - -.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE - - Change the disk image as if it had been created with *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 ``--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 ``--preallocation`` option may be used to specify - how the additional image area should be allocated on the host. See the format - description in the :ref:`notes` section which values are allowed. Using this - option may result in slightly more data being allocated than necessary. - -.. _notes: - -Notes ------ - -Supported image file formats: - -``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 *holes* (for example in ext2 or ext3 on - Linux or NTFS on Windows), then only the written sectors will reserve - space. Use ``qemu-img info`` to know the real size used by the - image or ``ls -ls`` on Unix/Linux. - - Supported options: - - ``preallocation`` - Preallocation mode (allowed values: ``off``, ``falloc``, - ``full``). ``falloc`` mode preallocates space for image by - calling ``posix_fallocate()``. ``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. - -``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: - - ``compat`` - Determines the qcow2 version to use. ``compat=0.10`` uses the - traditional image format that can be read by any QEMU since 0.10. - ``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. - - ``backing_file`` - File name of a base image (see ``create`` subcommand) - - ``backing_fmt`` - Image format of the base image - - ``encryption`` - If this option is set to ``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: - - - 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. - - - The user passphrase is directly used as the encryption key. A - poorly chosen or short passphrase will compromise the security - of the encryption. - - - 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. - - - 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. - - 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. - - ``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. - - ``preallocation`` - Preallocation mode (allowed values: ``off``, ``metadata``, - ``falloc``, ``full``). An image with preallocated metadata is - initially larger but can improve performance when the image needs - to grow. ``falloc`` and ``full`` preallocations are like the same - options of ``raw`` format, but sets up metadata also. - - ``lazy_refcounts`` - If this option is set to ``on``, reference count updates are - postponed with the goal of avoiding metadata I/O and improving - performance. This is particularly interesting with - ``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) - ``qemu-img check -r all`` is required, which may take some time. - - This option can only be enabled if ``compat=1.1`` is specified. - - ``nocow`` - If this option is set to ``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: - - - Disable it by mounting with nodatacow, then all newly created files - will be NOCOW - - 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 ``nocow=on``. One can - issue ``lsattr filename`` to check if the NOCOW flag is set or not - (Capital 'C' is NOCOW flag). - -``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 ``qemu-img --help``. For a more detailed - description of these formats, see the QEMU block drivers reference - 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. |