qemu-img(1)



QEMU-IMG.1(1)                                                    QEMU-IMG.1(1)

NAME
       qemu-img - 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
       Standard options:

       -h, --help
           Display this help and exit

       -V, --version
           Display version information and exit

       -T, --trace [[enable=]pattern][,events=file][,file=file]
           Specify tracing options.

           [enable=]pattern
               Immediately enable events matching pattern.  The file must
               contain one event name (as listed in the trace-events-all file)
               per line; globbing patterns are accepted too.  This option is
               only available if QEMU has been compiled with the simple, log
               or ftrace tracing backend.  To specify multiple events or
               patterns, specify the -trace option multiple times.

               Use "-trace help" to print a list of names of trace points.

           events=file
               Immediately enable events listed in file.  The file must
               contain one event name (as listed in the trace-events-all file)
               per line; globbing patterns are accepted too.  This option is
               only available if QEMU has been compiled with the simple, log
               or ftrace tracing backend.

           file=file
               Log output traces to file.  This option is only available if
               QEMU has been compiled with the simple tracing backend.

       The following commands are supported:

       amend [--object objectdef] [--image-opts] [-p] [-q] [-f fmt] [-t cache]
       -o options filename
       bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval]
       [-n] [--no-drain] [-o offset] [--pattern=pattern] [-q] [-s buffer_size]
       [-S step_size] [-t cache] [-w] [-U] filename
       check [--object objectdef] [--image-opts] [-q] [-f fmt] [--output=ofmt]
       [-r [leaks | all]] [-T src_cache] [-U] filename
       commit [--object objectdef] [--image-opts] [-q] [-f fmt] [-t cache] [-b
       base] [-d] [-p] filename
       compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T
       src_cache] [-p] [-q] [-s] [-U] filename1 filename2
       convert [--object objectdef] [--image-opts] [--target-image-opts] [-U]
       [-c] [-p] [-q] [-n] [-f fmt] [-t cache] [-T src_cache] [-O output_fmt]
       [-B backing_file] [-o options] [-s snapshot_id_or_name] [-l
       snapshot_param] [-S sparse_size] [-m num_coroutines] [-W] filename
       [filename2 [...]] output_filename
       create [--object objectdef] [-q] [-f fmt] [-b backing_file] [-F
       backing_fmt] [-u] [-o options] filename [size]
       dd [--image-opts] [-U] [-f fmt] [-O output_fmt] [bs=block_size]
       [count=blocks] [skip=blocks] if=input of=output
       info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt]
       [--backing-chain] [-U] filename
       map [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [-U]
       filename
       measure [--output=ofmt] [-O output_fmt] [-o options] [--size N |
       [--object objectdef] [--image-opts] [-f fmt] [-l snapshot_param]
       filename]
       snapshot [--object objectdef] [--image-opts] [-U] [-q] [-l | -a
       snapshot | -c snapshot | -d snapshot] filename
       rebase [--object objectdef] [--image-opts] [-U] [-q] [-f fmt] [-t
       cache] [-T src_cache] [-p] [-u] -b backing_file [-F backing_fmt]
       filename
       resize [--object objectdef] [--image-opts] [-q] [--shrink] filename [+
       | -]size

       Command parameters:

       filename
            is a disk image filename

       --object objectdef
           is a QEMU user creatable object definition. See the 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.

       --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.

       --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.

       fmt is the disk image format. It is guessed automatically in most
           cases. See below for a description of the supported disk formats.

       --backing-chain
           will enumerate information about backing files in a disk image
           chain. Refer below for further description.

       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]'

       snapshot_id_or_name
           is deprecated, use snapshot_param instead

       -c  indicates that target image must be compressed (qcow format only)

       -h  with or without a command shows help and lists the supported
           formats

       -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.

       -q  Quiet mode - do not print any output (except errors). There's no
           progress bar in case both -q and -p options are used.

       -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.

       -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.

       -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:

       snapshot
           is the name of the snapshot to create, apply or delete

       -a  applies a snapshot (revert disk to saved state)

       -c  creates a snapshot

       -d  deletes a snapshot

       -l  lists all snapshots in the given image

       Parameters to compare subcommand:

       -f  First image format

       -F  Second image format

       -s  Strict mode - fail on different image size or sector allocation

       Parameters to convert subcommand:

       -n  Skip the creation of the target volume

       -m  Number of parallel coroutines for the convert process

       -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.

       Parameters to dd subcommand:

       bs=block_size
           defines the block size

       count=blocks
           sets the number of input blocks to copy

       if=input
           sets the input file

       of=output
           sets the output file

       skip=blocks
           sets the number of input blocks to skip

       Command description:

       bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval]
       [-n] [--no-drain] [-o offset] [--pattern=pattern] [-q] [-s buffer_size]
       [-S step_size] [-t cache] [-w] 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 "-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.

       check [-f fmt] [--output=ofmt] [-r [leaks | all]] [-T src_cache]
       filename
           Perform a consistency check on the disk image filename. The command
           can output in the format ofmt which is either "human" or "json".

           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.

       create [-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.

       commit [-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).

       compare [-f fmt] [-F fmt] [-T src_cache] [-p] [-s] [-q] 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

       convert [-c] [-p] [-n] [-f fmt] [-t cache] [-T src_cache] [-O
       output_fmt] [-B backing_file] [-o options] [-s snapshot_id_or_name] [-l
       snapshot_param] [-m num_coroutines] [-W] [-S sparse_size] filename
       [filename2 [...]] output_filename
           Convert the disk image filename or a snapshot
           snapshot_param(snapshot_id_or_name is deprecated) 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).

       dd [-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 dd(1)'s size syntax.

       info [-f fmt] [--output=ofmt] [--backing-chain] 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. The command can output in the format
           ofmt which is either "human" or "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 "--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

       map [-f fmt] [--output=ofmt] 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.

       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".

           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.

       snapshot [-l | -a snapshot | -c snapshot | -d snapshot ] filename
           List, apply, create or delete snapshots in image filename.

       rebase [-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.

       resize [--shrink] [--preallocation=prealloc] 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 "NOTES" section which
           values are allowed.  Using this option may result in slightly more
           data being allocated than necessary.

       amend [-p] [-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.

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 zeros to underlying storage.

       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: 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 "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 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.

SEE ALSO
       The HTML documentation of QEMU for more precise information and Linux
       user mode emulator invocation.

AUTHOR
       Fabrice Bellard

                                  2018-01-11                     QEMU-IMG.1(1)

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