xref: /openbmc/qemu/docs/system/images.rst (revision f7160f32)
1.. _disk_005fimages:
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