qemu-img(1) - QEMU disk image utility



  • QEMU-IMG(1)							   QEMU-IMG(1)
    
    
    
    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
    
    
    
    				  2016-02-02			   QEMU-IMG(1)
    

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