| QEMU-IMG.1(1) | QEMU-IMG.1(1) |
qemu-img - QEMU disk image utility
qemu-img [standard options] 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.
Standard options:
Use "-trace help" to print a list of names of trace points.
The following commands are supported:
Command parameters:
Parameters to snapshot subcommand:
Parameters to compare subcommand:
Parameters to convert subcommand:
Parameters to dd subcommand:
Command description:
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 "-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.
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.
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:
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.
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 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:
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 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).
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.
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.
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
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:
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.
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
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 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:
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.
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.
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.
Supported image file formats:
Supported options:
Supported options:
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:
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.
This option can only be enabled if "compat=1.1" is specified.
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: a) Disable it by mounting with nodatacow, then all newly created files will be NOCOW. b) 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).
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 invocation.
Fabrice Bellard
| 2020-07-24 |