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  4. qemu-img(1)

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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:

Command parameters:

 is a disk image filename
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.
indicates that target image must be compressed (qcow format only)
with or without a command shows help and lists the supported formats
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.
Quiet mode - do not print any output (except errors). There's no progress bar in case both -q and -p options are used.
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.
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.
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:

is the name of the snapshot to create, apply or delete
applies a snapshot (revert disk to saved state)
creates a snapshot
deletes a snapshot
lists all snapshots in the given image

Parameters to compare subcommand:

First image format
Second image format
Strict mode - fail on on different image size or sector allocation

Parameters to convert subcommand:

Skip the creation of the target volume

Command description:

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 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 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.
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 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": 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.

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
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.

List, apply, create or delete snapshots in image 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).

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:

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.

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.

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.

Amends the image format specific options for the image file filename. Not all file formats support this operation.

NOTES

Supported image file formats:

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.
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.

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

"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.

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

2022-05-19