Introduce Virtio-loopback epsilon release:

Epsilon release introduces a new compatibility layer which make virtio-loopback
design to work with QEMU and rust-vmm vhost-user backend without require any
changes.

Signed-off-by: Timos Ampelikiotis <t.ampelikiotis@virtualopensystems.com>
Change-Id: I52e57563e08a7d0bdc002f8e928ee61ba0c53dd9

1 files changed, 85 insertions, 0 deletions

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+.. _disk images:
+
+Disk Images
+-----------
+
+QEMU supports many disk image formats, including growable disk images
+(their size increase as non empty sectors are written), compressed and
+encrypted disk images.
+
+.. _disk_005fimages_005fquickstart:
+
+Quick start for disk image creation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can create a disk image with the command::
+
+   qemu-img create myimage.img mysize
+
+where myimage.img is the disk image filename and mysize is its size in
+kilobytes. You can add an ``M`` suffix to give the size in megabytes and
+a ``G`` suffix for gigabytes.
+
+See the ``qemu-img`` invocation documentation for more information.
+
+.. _disk_005fimages_005fsnapshot_005fmode:
+
+Snapshot mode
+~~~~~~~~~~~~~
+
+If you use the option ``-snapshot``, all disk images are considered as
+read only. When sectors in written, they are written in a temporary file
+created in ``/tmp``. You can however force the write back to the raw
+disk images by using the ``commit`` monitor command (or C-a s in the
+serial console).
+
+.. _vm_005fsnapshots:
+
+VM snapshots
+~~~~~~~~~~~~
+
+VM snapshots are snapshots of the complete virtual machine including CPU
+state, RAM, device state and the content of all the writable disks. In
+order to use VM snapshots, you must have at least one non removable and
+writable block device using the ``qcow2`` disk image format. Normally
+this device is the first virtual hard drive.
+
+Use the monitor command ``savevm`` to create a new VM snapshot or
+replace an existing one. A human readable name can be assigned to each
+snapshot in addition to its numerical ID.
+
+Use ``loadvm`` to restore a VM snapshot and ``delvm`` to remove a VM
+snapshot. ``info snapshots`` lists the available snapshots with their
+associated information::
+
+   (qemu) info snapshots
+   Snapshot devices: hda
+   Snapshot list (from hda):
+   ID        TAG                 VM SIZE                DATE       VM CLOCK
+   1         start                   41M 2006-08-06 12:38:02   00:00:14.954
+   2                                 40M 2006-08-06 12:43:29   00:00:18.633
+   3         msys                    40M 2006-08-06 12:44:04   00:00:23.514
+
+A VM snapshot is made of a VM state info (its size is shown in
+``info snapshots``) and a snapshot of every writable disk image. The VM
+state info is stored in the first ``qcow2`` non removable and writable
+block device. The disk image snapshots are stored in every disk image.
+The size of a snapshot in a disk image is difficult to evaluate and is
+not shown by ``info snapshots`` because the associated disk sectors are
+shared among all the snapshots to save disk space (otherwise each
+snapshot would need a full copy of all the disk images).
+
+When using the (unrelated) ``-snapshot`` option
+(:ref:`disk_005fimages_005fsnapshot_005fmode`),
+you can always make VM snapshots, but they are deleted as soon as you
+exit QEMU.
+
+VM snapshots currently have the following known limitations:
+
+-  They cannot cope with removable devices if they are removed or
+   inserted after a snapshot is done.
+
+-  A few device drivers still have incomplete snapshot support so their
+   state is not saved or restored properly (in particular USB).
+
+.. include:: qemu-block-drivers.rst.inc