qemu/docs/system/images.rst
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   1.. _disk images:
   2
   3Disk Images
   4-----------
   5
   6QEMU supports many disk image formats, including growable disk images
   7(their size increase as non empty sectors are written), compressed and
   8encrypted disk images.
   9
  10.. _disk_005fimages_005fquickstart:
  11
  12Quick start for disk image creation
  13~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  14
  15You can create a disk image with the command::
  16
  17   qemu-img create myimage.img mysize
  18
  19where myimage.img is the disk image filename and mysize is its size in
  20kilobytes. You can add an ``M`` suffix to give the size in megabytes and
  21a ``G`` suffix for gigabytes.
  22
  23See the qemu-img invocation documentation for more information.
  24
  25.. _disk_005fimages_005fsnapshot_005fmode:
  26
  27Snapshot mode
  28~~~~~~~~~~~~~
  29
  30If you use the option ``-snapshot``, all disk images are considered as
  31read only. When sectors in written, they are written in a temporary file
  32created in ``/tmp``. You can however force the write back to the raw
  33disk images by using the ``commit`` monitor command (or C-a s in the
  34serial console).
  35
  36.. _vm_005fsnapshots:
  37
  38VM snapshots
  39~~~~~~~~~~~~
  40
  41VM snapshots are snapshots of the complete virtual machine including CPU
  42state, RAM, device state and the content of all the writable disks. In
  43order to use VM snapshots, you must have at least one non removable and
  44writable block device using the ``qcow2`` disk image format. Normally
  45this device is the first virtual hard drive.
  46
  47Use the monitor command ``savevm`` to create a new VM snapshot or
  48replace an existing one. A human readable name can be assigned to each
  49snapshot in addition to its numerical ID.
  50
  51Use ``loadvm`` to restore a VM snapshot and ``delvm`` to remove a VM
  52snapshot. ``info snapshots`` lists the available snapshots with their
  53associated information::
  54
  55   (qemu) info snapshots
  56   Snapshot devices: hda
  57   Snapshot list (from hda):
  58   ID        TAG                 VM SIZE                DATE       VM CLOCK
  59   1         start                   41M 2006-08-06 12:38:02   00:00:14.954
  60   2                                 40M 2006-08-06 12:43:29   00:00:18.633
  61   3         msys                    40M 2006-08-06 12:44:04   00:00:23.514
  62
  63A VM snapshot is made of a VM state info (its size is shown in
  64``info snapshots``) and a snapshot of every writable disk image. The VM
  65state info is stored in the first ``qcow2`` non removable and writable
  66block device. The disk image snapshots are stored in every disk image.
  67The size of a snapshot in a disk image is difficult to evaluate and is
  68not shown by ``info snapshots`` because the associated disk sectors are
  69shared among all the snapshots to save disk space (otherwise each
  70snapshot would need a full copy of all the disk images).
  71
  72When using the (unrelated) ``-snapshot`` option
  73(:ref:`disk_005fimages_005fsnapshot_005fmode`),
  74you can always make VM snapshots, but they are deleted as soon as you
  75exit QEMU.
  76
  77VM snapshots currently have the following known limitations:
  78
  79-  They cannot cope with removable devices if they are removed or
  80   inserted after a snapshot is done.
  81
  82-  A few device drivers still have incomplete snapshot support so their
  83   state is not saved or restored properly (in particular USB).
  84
  85.. include:: qemu-block-drivers.rst.inc
  86