Technical Notes

Manpages - qemu-img.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 (either event name or a globbing pattern). 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] [–force] -o OPTIONS FILENAME ::
  • *bench [-c COUNT] [-d DEPTH] [-f FMT] [–flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [–no-drain] [-o OFFSET] [–pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME* ::
  • *bitmap (–merge SOURCE | –add | –remove | –clear | –enable | –disable)… [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [–object OBJECTDEF] [–image-opts | -f FMT] FILENAME BITMAP* ::
  • 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] [-r RATE_LIMIT] [-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] [–target-is-zero] [–bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] [–salvage] 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] [–start-offset=OFFSET] [–max-length=LEN] [–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] [-c] -b BACKING_FILE [-F BACKING_FMT] FILENAME* ::
  • resize [–object OBJECTDEF] [–image-opts] [-f FMT] [–preallocation=PREALLOC] [-q] [–shrink] FILENAME [+ | -]SIZE ::

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.

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 help 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]'.

–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.
–force-share (-U)
If specified, qemu-img will open the image in shared mode, allowing other QEMU processes to open it in write mode. For example, this can be used to get the image information (with 'info' subcommand) when the image is used by a running guest. Note that this could produce inconsistent results because of concurrent metadata changes, etc. This option is only allowed when opening images in read-only mode.
–backing-chain
Will enumerate information about backing files in a disk image chain. Refer below for further description.
-c
Indicates that target image must be compressed (qcow/qcow2 and vmdk with streamOptimized subformat only). For qcow2, the compression algorithm can be specified with the -o compression_type=… option (see below).
-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 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:

–bitmaps
Additionally copy all persistent bitmaps from the top layer of the source
-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.
-C
Try to use copy offloading to move data from source image to target. This may improve performance if the data is remote, such as with NFS or iSCSI backends, but will not automatically sparsify zero sectors, and may result in a fully allocated target image depending on the host support for getting allocation information.
-r
Rate limit for the convert process
–salvage
Try to ignore I/O errors when reading. Unless in quiet mode (-q), errors will still be printed. Areas that cannot be read from the source will be treated as containing only zeroes.
–target-is-zero
Assume that reading the destination image will always return zeros. This parameter is mutually exclusive with a destination image that has a backing file. It is required to also use the -n parameter to skip image creation.

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

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

Command description:

  • amend [–object OBJECTDEF] [–image-opts] [-p] [-q] [-f FMT] [-t CACHE] [–force] -o OPTIONS FILENAME :: Amends the image format specific OPTIONS for the image file FILENAME. Not all file formats support this operation. The set of options that can be amended are dependent on the image format, but note that amending the backing chain relationship should instead be performed with qemu-img rebase. –force allows some unsafe operations. Currently for -f luks, it allows to erase the last encryption key, and to overwrite an active encryption key.
  • *bench [-c COUNT] [-d DEPTH] [-f FMT] [–flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [–no-drain] [-o OFFSET] [–pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] 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 -i is specified, AIO option can be used to specify different AIO backends: threads, native or io_uring. 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.
  • *bitmap (–merge SOURCE | –add | –remove | –clear | –enable | –disable)… [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [–object OBJECTDEF] [–image-opts | -f FMT] FILENAME BITMAP* :: Perform one or more modifications of the persistent bitmap BITMAP in the disk image FILENAME. The various modifications are: –add to create BITMAP, enabled to record future edits. –remove to remove BITMAP. –clear to clear BITMAP. –enable to change BITMAP to start recording future edits. –disable to change BITMAP to stop recording future edits. –merge to merge the contents of the SOURCE bitmap into BITMAP. Additional options include -g which sets a non-default GRANULARITY for –add, and -b and -F which select an alternative source file for all SOURCE bitmaps used by –merge. To see what bitmaps are present in an image, use qemu-img info.

  • *check [–object OBJECTDEF] [–image-opts] [-q] [-f FMT] [–output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME* :: Perform a consistency check on the disk image FILENAME. The command can output in the format OFMT which is either human or json. The JSON output is an object of QAPI type ImageCheck.

    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, parallels, vhdx, vmdk 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.

  • commit [–object OBJECTDEF] [–image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-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). The rate limit for the commit process is specified by -r.

  • compare [–object OBJECTDEF] [–image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] 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 summarizes all exit codes of the compare subcommand:

    0
    Images are identical (or requested help was printed)
    1
    Images differ
    2
    Error on opening an image
    3
    Error on checking a sector allocation
    4
    Error on reading data
  • *convert [–object OBJECTDEF] [–image-opts] [–target-image-opts] [–target-is-zero] [–bitmaps [–skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 […]] OUTPUT_FILENAME* :: Convert the disk image FILENAME or a snapshot SNAPSHOT_PARAM 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 (as given by BACKING_FMT), 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). Use of –bitmaps requests that any persistent bitmaps present in the original are also copied to the destination. If any bitmap is inconsistent in the source, the conversion will fail unless –skip-broken-bitmaps is also specified to copy only the consistent bitmaps.
  • create [–object OBJECTDEF] [-q] [-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.
  • dd [–image-opts] [-U] [-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 [–object OBJECTDEF] [–image-opts] [-f FMT] [–output=OFMT] [–backing-chain] [-U] 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.

    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

The command can output in the format OFMT which is either human or json. The JSON output is an object of QAPI type ImageInfo; with –backing-chain, it is an array of ImageInfo objects.

–output=human reports the following information (for every image in the chain):

image
The image file name
file format
The image format
virtual size
The size of the guest disk
disk size
How much space the image file occupies on the host file system (may be shown as 0 if this information is unavailable, e.g. because there is no file system)
cluster_size
Cluster size of the image format, if applicable
encrypted
Whether the image is encrypted (only present if so)
cleanly shut down
This is shown as no if the image is dirty and will have to be auto-repaired the next time it is opened in qemu.
backing file
The backing file name, if present
backing file format
The format of the backing file, if the image enforces it
Snapshot list
A list of all internal snapshots
Format specific information
Further information whose structure depends on the image format. This section is a textual representation of the respective ImageInfoSpecific* QAPI object (e.g. ImageInfoSpecificQCow2 for qcow2 images).

  • *map [–object OBJECTDEF] [–image-opts] [-f FMT] [–start-offset=OFFSET] [–max-length=LEN] [–output=OFMT] [-U] 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:

  • boolean field data: true if the sectors contain actual data, false if the sectors are either unallocated or stored as optimized all-zero clusters
  • boolean field zero: true if the data is known to read as zero
  • boolean field present: true if the data belongs to the backing chain, false if rebasing the backing chain onto a deeper file would pick up data from the deeper file;
  • integer field depth: the depth within the backing chain at which the data was resolved; 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. The JSON output is an object of QAPI type BlockMeasureInfo.

    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
bitmaps size: 0

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.

The bitmaps size is the additional size required in order to copy bitmaps from a source image in addition to the guest-visible data; the line is omitted if either source or destination lacks bitmap support, or 0 if bitmaps are supported but there is nothing to copy.

  • snapshot [–object OBJECTDEF] [–image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME :: List, apply, create or delete snapshots in image FILENAME.

  • *rebase [–object OBJECTDEF] [–image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] [-c] -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. With the -c option specified, the clusters which are being merged (but not the entire FILENAME image) are compressed when written. 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 [–object OBJECTDEF] [–image-opts] [-f FMT] [–preallocation=PREALLOC] [-q] [–shrink] 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.

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 data to underlying storage. This data may or may not be zero, depending on the storage location.

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 or zstd 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
compression_type
This option configures which compression algorithm will be used for compressed clusters on the image. Note that setting this option doesn't yet cause the image to actually receive compressed writes. It is most commonly used with the -c option of qemu-img convert, but can also be used with the compress filter driver or backup block jobs with compression enabled. Valid values are zlib and zstd. For images that use compat=0.10, only zlib compression is available.
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:

  • Disable it by mounting with nodatacow, then all newly created files will be NOCOW
  • 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).

data_file
Filename where all guest data will be stored. If this option is used, the qcow2 file will only contain the image's metadata. Note: Data loss will occur if the given filename already exists when using this option with qemu-img create since qemu-img will create the data file anew, overwriting the file's original contents. To simply update the reference to point to the given pre-existing file, use qemu-img amend.
data_file_raw
If this option is set to on, QEMU will always keep the external data file consistent as a standalone read-only raw image. It does this by forwarding all write accesses to the qcow2 file through to the raw data file, including their offsets. Therefore, data that is visible on the qcow2 node (i.e., to the guest) at some offset is visible at the same offset in the raw data file. This results in a read-only raw image. Writes that bypass the qcow2 metadata may corrupt the qcow2 metadata because the out-of-band writes may result in the metadata falling out of sync with the raw image. If this option is off, QEMU will use the data file to store data in an arbitrary manner. The file's content will not make sense without the accompanying qcow2 metadata. Where data is written will have no relation to its offset as seen by the guest, and some writes (specifically zero writes) may not be forwarded to the data file at all, but will only be handled by modifying qcow2 metadata. This option can only be enabled if data_file is set.

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 block drivers reference 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.

AUTHOR

Fabrice Bellard

COPYRIGHT

2024, The QEMU Project Developers