trusty (1) qemu-img.1.gz

Provided by: qemu-utils_2.0.0+dfsg-2ubuntu1.46_amd64 bug

NAME

       qemu-img - QEMU disk image utility

SYNOPSIS

       usage: qemu-img 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

       The following commands are supported:

       check [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]] filename
       create [-q] [-f fmt] [-o options] filename [size]
       commit [-q] [-f fmt] [-t cache] filename
       compare [-f fmt] [-F fmt] [-p] [-q] [-s] filename1 filename2
       convert [-c] [-p] [-q] [-n] [-f fmt] [-t cache] [-O output_fmt] [-o options] [-s snapshot_id_or_name] [-l
       snapshot_param] [-S sparse_size] filename [filename2 [...]] output_filename
       info [-f fmt] [--output=ofmt] [--backing-chain] filename
       map [-f fmt] [--output=ofmt] filename
       snapshot [-q] [-l | -a snapshot | -c snapshot | -d snapshot] filename
       rebase [-q] [-f fmt] [-t cache] [-p] [-u] -b backing_file [-F backing_fmt] filename
       resize [-q] filename [+ | -]size
       amend [-q] [-f fmt] -o options filename

       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.

       --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" 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.

       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 on different image size or sector allocation

       Parameters to convert subcommand:

       -n  Skip the creation of the target volume

       Command description:

       check [-f fmt] [--output=ofmt] [-r [leaks | all]] 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.

       create [-f fmt] [-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).

           The size can also be specified using the size option with "-o", it doesn't need to be specified
           separately in this case.

       commit [-f fmt] [-t cache] 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.

       compare [-f fmt] [-F fmt] [-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] [-O output_fmt] [-o options] [-s snapshot_id_or_name] [-l
       snapshot_param] [-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" or
           "cow": 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 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.

       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.

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

       rebase [-f fmt] [-t 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).

           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 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!

           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.

       amend [-f fmt] -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.

       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.

               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). An image with preallocated metadata is
               initially larger but can improve performance when the image needs to grow.

           "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.

       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

                                                   2019-05-09                                        QEMU-IMG(1)