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CentOS 7.0 - man page for qemu-img (centos section 1)

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

       qemu-img - QEMU disk image utility

       usage: qemu-img command [command options]

       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.

       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_name] [-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:

	    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.

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

	   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.

	   is the destination disk image filename

	    is the destination format

	   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.

       -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:

	   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

	   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

	   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_name]
       [-S sparse_size] filename [filename2 [...]] output_filename
	   Convert the disk image filename or a snapshot snapshot_name 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

	   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

	   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.

       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.

	   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:

	       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

	       File name of a base image (see create subcommand)

	       Image format of the base image

	       If this option is set to "on", the image is encrypted.

	       Encryption uses the AES format which is very secure (128 bit keys). Use a long
	       password (16 characters) to get maximum protection.

	       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 mode (allowed values: off, metadata). An image with preallocated
	       metadata is initially larger but can improve performance when the image needs to

	       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.

	   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.

       The HTML documentation of QEMU for more precise information and Linux user mode emulator

       Fabrice Bellard

					    2014-06-10				      QEMU-IMG(1)

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