2020-11-16 17:47:36 +03:00
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.. _disk images:
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2020-02-28 18:36:05 +03:00
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Disk Images
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-----------
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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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.. _disk_005fimages_005fquickstart:
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Quick start for disk image creation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You can create a disk image with the command::
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qemu-img create myimage.img mysize
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where myimage.img is the disk image filename and mysize is its size in
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kilobytes. You can add an ``M`` suffix to give the size in megabytes and
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a ``G`` suffix for gigabytes.
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See the qemu-img invocation documentation for more information.
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.. _disk_005fimages_005fsnapshot_005fmode:
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Snapshot mode
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~~~~~~~~~~~~~
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If you use the option ``-snapshot``, all disk images are considered as
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read only. When sectors in written, they are written in a temporary file
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created in ``/tmp``. You can however force the write back to the raw
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disk images by using the ``commit`` monitor command (or C-a s in the
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serial console).
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.. _vm_005fsnapshots:
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VM snapshots
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~~~~~~~~~~~~
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VM snapshots are snapshots of the complete virtual machine including CPU
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state, RAM, device state and the content of all the writable disks. In
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order to use VM snapshots, you must have at least one non removable and
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writable block device using the ``qcow2`` disk image format. Normally
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this device is the first virtual hard drive.
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Use the monitor command ``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 ``loadvm`` to restore a VM snapshot and ``delvm`` to remove a VM
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snapshot. ``info snapshots`` lists the available snapshots with their
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associated information::
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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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A VM snapshot is made of a VM state info (its size is shown in
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``info snapshots``) and a snapshot of every writable disk image. The VM
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state info is stored in the first ``qcow2`` non removable and writable
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block device. The disk image snapshots are stored in every disk image.
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The size of a snapshot in a disk image is difficult to evaluate and is
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not shown by ``info snapshots`` because the associated disk sectors are
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shared among all the snapshots to save disk space (otherwise each
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snapshot would need a full copy of all the disk images).
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When using the (unrelated) ``-snapshot`` option
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(:ref:`disk_005fimages_005fsnapshot_005fmode`),
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you can always make VM snapshots, but they are deleted as soon as you
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exit QEMU.
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VM snapshots currently have the following known limitations:
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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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- 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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.. include:: qemu-block-drivers.rst.inc
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