89 lines
3.4 KiB
Plaintext
89 lines
3.4 KiB
Plaintext
@node disk_images
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@section Disk Images
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QEMU supports many disk image formats, including growable disk images
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(their size increase as non empty sectors are written), compressed and
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encrypted disk images.
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@menu
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* disk_images_quickstart:: Quick start for disk image creation
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* disk_images_snapshot_mode:: Snapshot mode
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* vm_snapshots:: VM snapshots
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@end menu
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@node disk_images_quickstart
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@subsection Quick start for disk image creation
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You can create a disk image with the command:
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@example
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qemu-img create myimage.img mysize
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@end example
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where @var{myimage.img} is the disk image filename and @var{mysize} is its
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size in kilobytes. You can add an @code{M} suffix to give the size in
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megabytes and a @code{G} suffix for gigabytes.
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@c When this document is converted to rst we should make this into
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@c a proper linked reference to the qemu-img documentation again:
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See the qemu-img invocation documentation for more information.
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@node disk_images_snapshot_mode
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@subsection Snapshot mode
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If you use the option @option{-snapshot}, all disk images are
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considered as read only. When sectors in written, they are written in
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a temporary file created in @file{/tmp}. You can however force the
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write back to the raw disk images by using the @code{commit} monitor
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command (or @key{C-a s} in the serial console).
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@node vm_snapshots
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@subsection VM snapshots
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VM snapshots are snapshots of the complete virtual machine including
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CPU state, RAM, device state and the content of all the writable
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disks. In order to use VM snapshots, you must have at least one non
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removable and writable block device using the @code{qcow2} disk image
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format. Normally this device is the first virtual hard drive.
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Use the monitor command @code{savevm} to create a new VM snapshot or
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replace an existing one. A human readable name can be assigned to each
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snapshot in addition to its numerical ID.
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Use @code{loadvm} to restore a VM snapshot and @code{delvm} to remove
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a VM snapshot. @code{info snapshots} lists the available snapshots
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with their associated information:
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@example
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(qemu) info snapshots
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Snapshot devices: hda
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Snapshot list (from hda):
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ID TAG VM SIZE DATE VM CLOCK
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1 start 41M 2006-08-06 12:38:02 00:00:14.954
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2 40M 2006-08-06 12:43:29 00:00:18.633
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3 msys 40M 2006-08-06 12:44:04 00:00:23.514
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@end example
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A VM snapshot is made of a VM state info (its size is shown in
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@code{info snapshots}) and a snapshot of every writable disk image.
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The VM state info is stored in the first @code{qcow2} non removable
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and writable block device. The disk image snapshots are stored in
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every disk image. The size of a snapshot in a disk image is difficult
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to evaluate and is not shown by @code{info snapshots} because the
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associated disk sectors are shared among all the snapshots to save
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disk space (otherwise each snapshot would need a full copy of all the
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disk images).
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When using the (unrelated) @code{-snapshot} option
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(@ref{disk_images_snapshot_mode}), you can always make VM snapshots,
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but they are deleted as soon as you exit QEMU.
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VM snapshots currently have the following known limitations:
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@itemize
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@item
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They cannot cope with removable devices if they are removed or
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inserted after a snapshot is done.
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@item
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A few device drivers still have incomplete snapshot support so their
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state is not saved or restored properly (in particular USB).
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@end itemize
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