@item -h
with or without a command shows help and lists the supported formats
@item -p
-display progress bar (convert and rebase commands only)
+display progress bar (compare, convert and rebase commands only).
+If the @var{-p} option is not used for a command that supports it, the
+progress is reported when the process receives a @code{SIGUSR1} signal.
@item -q
Quiet mode - do not print any output (except errors). There's no progress bar
in case both @var{-q} and @var{-p} options are used.
@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}
-Commit the changes recorded in @var{filename} in its base image.
+Commit the changes recorded in @var{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.
@item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2}
Supported options:
@table @code
@item compat
-Determines the qcow2 version to use. @code{compat=0.10} uses the traditional
-image format that can be read by any QEMU since 0.10 (this is the default).
+Determines the qcow2 version to use. @code{compat=0.10} uses the
+traditional image format that can be read by any QEMU since 0.10.
@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
-newer understand. Amongst others, this includes zero clusters, which allow
-efficient copy-on-read for sparse images.
+newer understand (this is the default). Amongst others, this includes zero
+clusters, which allow efficient copy-on-read for sparse images.
@item backing_file
File name of a base image (see @option{create} subcommand)
@item backing_fmt
Image format of the base image
@item encryption
-If this option is set to @code{on}, the image is encrypted.
+If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
-Encryption uses the AES format which is very secure (128 bit keys). Use
-a long password (16 characters) to get maximum protection.
+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:
+
+@itemize @minus
+@item 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.
+@item The user passphrase is directly used as the encryption key. A poorly
+chosen or short passphrase will compromise the security of the encryption.
+@item 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.
+@end itemize
+
+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.
@item cluster_size
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
@item Other
QEMU also supports various other image file formats for compatibility with
-older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), qcow1
-and QED. For a full list of supported formats see @code{qemu-img --help}.
+older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
+qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
For a more detailed description of these formats, see the QEMU Emulation User
Documentation.