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Documentation: Update image format information
Document new and yet undocumented options and image formats. The qemu-img man page contains information only for raw and qcow2 now and references the HTML documentation for a more detailed description of other formats. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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167
qemu-doc.texi
167
qemu-doc.texi
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@ -416,6 +416,7 @@ snapshots.
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* vm_snapshots:: VM snapshots
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* qemu_img_invocation:: qemu-img Invocation
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* qemu_nbd_invocation:: qemu-nbd Invocation
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* disk_images_formats:: Disk image file formats
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* host_drives:: Using host drives
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* disk_images_fat_images:: Virtual FAT disk images
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* disk_images_nbd:: NBD access
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@ -507,6 +508,172 @@ state is not saved or restored properly (in particular USB).
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@include qemu-nbd.texi
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@node disk_images_formats
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@subsection Disk image file formats
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QEMU supports many image file formats that can be used with VMs as well as with
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any of the tools (like @code{qemu-img}). This includes the preferred formats
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raw and qcow2 as well as formats that are supported for compatibility with
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older QEMU versions or other hypervisors.
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Depending on the image format, different options can be passed to
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@code{qemu-img create} and @code{qemu-img convert} using the @code{-o} option.
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This section describes each format and the options that are supported for it.
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@table @option
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@item raw
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Raw disk image format. This format has the advantage of
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being simple and easily exportable to all other emulators. If your
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file system supports @emph{holes} (for example in ext2 or ext3 on
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Linux or NTFS on Windows), then only the written sectors will reserve
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space. Use @code{qemu-img info} to know the real size used by the
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image or @code{ls -ls} on Unix/Linux.
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@item qcow2
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QEMU image format, the most versatile format. Use it to have smaller
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images (useful if your filesystem does not supports holes, for example
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on Windows), optional AES encryption, zlib based compression and
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support of multiple VM snapshots.
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Supported options:
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@table @code
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@item compat
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Determines the qcow2 version to use. @code{compat=0.10} uses the traditional
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image format that can be read by any QEMU since 0.10 (this is the default).
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@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
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newer understand. Amongst others, this includes zero clusters, which allow
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efficient copy-on-read for sparse images.
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@item backing_file
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File name of a base image (see @option{create} subcommand)
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@item backing_fmt
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Image format of the base image
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@item encryption
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If this option is set to @code{on}, the image is encrypted.
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Encryption uses the AES format which is very secure (128 bit keys). Use
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a long password (16 characters) to get maximum protection.
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@item cluster_size
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Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
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sizes can improve the image file size whereas larger cluster sizes generally
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provide better performance.
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@item preallocation
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Preallocation mode (allowed values: off, metadata). An image with preallocated
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metadata is initially larger but can improve performance when the image needs
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to grow.
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@item lazy_refcounts
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If this option is set to @code{on}, reference count updates are postponed with
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the goal of avoiding metadata I/O and improving performance. This is
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particularly interesting with @option{cache=writethrough} which doesn't batch
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metadata updates. The tradeoff is that after a host crash, the reference count
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tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
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check -r all} is required, which may take some time.
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This option can only be enabled if @code{compat=1.1} is specified.
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@end table
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@item qed
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Old QEMU image format with support for backing files and compact image files
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(when your filesystem or transport medium does not support holes).
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When converting QED images to qcow2, you might want to consider using the
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@code{lazy_refcounts=on} option to get a more QED-like behaviour.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand).
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@item backing_fmt
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Image file format of backing file (optional). Useful if the format cannot be
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autodetected because it has no header, like some vhd/vpc files.
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@item cluster_size
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Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller
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cluster sizes can improve the image file size whereas larger cluster sizes
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generally provide better performance.
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@item table_size
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Changes the number of clusters per L1/L2 table (must be power-of-2 between 1
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and 16). There is normally no need to change this value but this option can be
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used for performance benchmarking.
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@end table
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@item qcow
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Old QEMU image format with support for backing files, compact image files,
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encryption and compression.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand)
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@item encryption
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If this option is set to @code{on}, the image is encrypted.
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@end table
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@item cow
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User Mode Linux Copy On Write image format. It is supported only for
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compatibility with previous versions.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand)
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@end table
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@item vdi
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VirtualBox 1.1 compatible image format.
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Supported options:
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@table @code
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@item static
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If this option is set to @code{on}, the image is created with metadata
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preallocation.
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@end table
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@item vmdk
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VMware 3 and 4 compatible image format.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand).
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@item compat6
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Create a VMDK version 6 image (instead of version 4)
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@item subformat
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Specifies which VMDK subformat to use. Valid options are
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@code{monolithicSparse} (default),
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@code{monolithicFlat},
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@code{twoGbMaxExtentSparse},
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@code{twoGbMaxExtentFlat} and
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@code{streamOptimized}.
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@end table
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@item vpc
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VirtualPC compatible image format (VHD).
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Supported options:
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@table @code
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@item subformat
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Specifies which VHD subformat to use. Valid options are
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@code{dynamic} (default) and @code{fixed}.
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@end table
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@end table
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@subsubsection Read-only formats
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More disk image file formats are supported in a read-only mode.
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@table @option
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@item bochs
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Bochs images of @code{growing} type.
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@item cloop
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Linux Compressed Loop image, useful only to reuse directly compressed
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CD-ROM images present for example in the Knoppix CD-ROMs.
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@item dmg
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Apple disk image.
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@item parallels
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Parallels disk image format.
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@end table
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@node host_drives
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@subsection Using host drives
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@ -226,7 +226,10 @@ After using this command to grow a disk image, you must use file system and
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partitioning tools inside the VM to actually begin using the new space on the
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device.
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@end table
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@c man end
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@ignore
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@c man begin NOTES
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Supported image file formats:
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@table @option
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@ -247,6 +250,13 @@ support of multiple VM snapshots.
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Supported options:
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@table @code
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@item compat
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Determines the qcow2 version to use. @code{compat=0.10} uses the traditional
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image format that can be read by any QEMU since 0.10 (this is the default).
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@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
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newer understand. Amongst others, this includes zero clusters, which allow
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efficient copy-on-read for sparse images.
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@item backing_file
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File name of a base image (see @option{create} subcommand)
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@item backing_fmt
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@ -267,73 +277,33 @@ Preallocation mode (allowed values: off, metadata). An image with preallocated
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metadata is initially larger but can improve performance when the image needs
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to grow.
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@item lazy_refcounts
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If this option is set to @code{on}, reference count updates are postponed with
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the goal of avoiding metadata I/O and improving performance. This is
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particularly interesting with @option{cache=writethrough} which doesn't batch
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metadata updates. The tradeoff is that after a host crash, the reference count
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tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
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check -r all} is required, which may take some time.
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This option can only be enabled if @code{compat=1.1} is specified.
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@end table
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@item qed
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Image format with support for backing files and compact image files (when your
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filesystem or transport medium does not support holes). Good performance due
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to less metadata than the more featureful qcow2 format, especially with
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cache=writethrough or cache=directsync. Consider using qcow2 which will soon
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have a similar optimization and is most actively developed.
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@item Other
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QEMU also supports various other image file formats for compatibility with
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older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), qcow1
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and QED. For a full list of supported formats see @code{qemu-img --help}.
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For a more detailed description of these formats, see the QEMU Emulation User
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Documentation.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand).
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@item backing_fmt
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Image file format of backing file (optional). Useful if the format cannot be
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autodetected because it has no header, like some vhd/vpc files.
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@item cluster_size
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Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller
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cluster sizes can improve the image file size whereas larger cluster sizes
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generally provide better performance.
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@item table_size
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Changes the number of clusters per L1/L2 table (must be power-of-2 between 1
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and 16). There is normally no need to change this value but this option can be
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used for performance benchmarking.
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@end table
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@item qcow
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Old QEMU image format. Left for compatibility.
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Supported options:
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@table @code
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@item backing_file
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File name of a base image (see @option{create} subcommand)
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@item encryption
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If this option is set to @code{on}, the image is encrypted.
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@end table
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@item cow
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User Mode Linux Copy On Write image format. Used to be the only growable
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image format in QEMU. It is supported only for compatibility with
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previous versions. It does not work on win32.
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@item vdi
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VirtualBox 1.1 compatible image format.
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@item vmdk
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VMware 3 and 4 compatible image format.
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Supported options:
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@table @code
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@item backing_fmt
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Image format of the base image
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@item compat6
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Create a VMDK version 6 image (instead of version 4)
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@end table
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@item vpc
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VirtualPC compatible image format (VHD).
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@item cloop
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Linux Compressed Loop image, useful only to reuse directly compressed
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CD-ROM images present for example in the Knoppix CD-ROMs.
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The main purpose of the block drivers for these formats is image conversion.
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For running VMs, it is recommended to convert the disk images to either raw or
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qcow2 in order to achieve good performance.
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@end table
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@c man end
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@ignore
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@setfilename qemu-img
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@settitle QEMU disk image utility
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