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readme: qemu snapshot is stored in qcow2, qemu-img info

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Ciro Santilli
2018-04-27 21:25:52 +01:00
parent 41aac40759
commit 0b9157c919
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README.adoc
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@@ -573,6 +573,8 @@ the disk image gets overwritten by a fresh filesystem and you lose all changes.
Remember that if you forcibly turn QEMU off without `sync` or `poweroff` from inside the VM, e.g. by closing the QEMU window, disk changes may not be saved. Remember that if you forcibly turn QEMU off without `sync` or `poweroff` from inside the VM, e.g. by closing the QEMU window, disk changes may not be saved.
Persistency can be turned off by passing the `snapshot` option to `-drive` (not exposed on our scripts).
When booting from <<initrd>> however without a disk, persistency is lost. When booting from <<initrd>> however without a disk, persistency is lost.
=== Kernel command line parameters === Kernel command line parameters
@@ -1792,8 +1794,8 @@ Scripts under `/etc/init.d` are run by `/etc/init.d/rcS`, which gets called by t
The init is selected at: The init is selected at:
- initrd or initramfs system: `/init`, a custom one can be set with the `rdinit=` <<kernel-command-line-parameters,kernel command line parameter>> * initrd or initramfs system: `/init`, a custom one can be set with the `rdinit=` <<kernel-command-line-parameters,kernel command line parameter>>
- otherwise: default is `/sbin/init`, followed by some other paths, a custom one can be set with `init=` * otherwise: default is `/sbin/init`, followed by some other paths, a custom one can be set with `init=`
More details: https://unix.stackexchange.com/questions/30414/what-can-make-passing-init-path-to-program-to-the-kernel-not-start-program-as-i/430614#430614 More details: https://unix.stackexchange.com/questions/30414/what-can-make-passing-init-path-to-program-to-the-kernel-not-start-program-as-i/430614#430614
@@ -3015,7 +3017,7 @@ The counting continues.
Restore the snapshot: Restore the snapshot:
.... ....
echo 'loadvm my_snap_id' | ./qemumonitor echo 'loadvm my_snap_id' | ./qemumonitor
.... ....
and the counting goes back to where we saved. This shows that CPU and memory states were reverted. and the counting goes back to where we saved. This shows that CPU and memory states were reverted.
@@ -3060,6 +3062,40 @@ And the output is `0`.
Our setup does not allow for snapshotting while using <<initrd>>. Our setup does not allow for snapshotting while using <<initrd>>.
==== Snapshot internals
Snapshots are stored inside the `.qcow2` images themselves.
They can be observed with:
....
./out/x86_64/buildroot/host/bin/qemu-img info out/x86_64/buildroot/images/rootfs.ext2.qcow2
....
which after `savevm my_snap_id` and `savevm asdf` contains an output of type:
....
image: out/x86_64/buildroot/images/rootfs.ext2.qcow2
file format: qcow2
virtual size: 512M (536870912 bytes)
disk size: 180M
cluster_size: 65536
Snapshot list:
ID TAG VM SIZE DATE VM CLOCK
1 my_snap_id 47M 2018-04-27 21:17:50 00:00:15.251
2 asdf 47M 2018-04-27 21:20:39 00:00:18.583
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
....
As a consequence:
* it is possible to restore snapshots across boots, since they stay on the same image the entire time
* it is not possible to use snapshots with <<initrd>> in our setup, since we don't pass `-drive` at all when initrd is enabled
=== Educational hardware models === Educational hardware models
We have added and interacted with a few educational hardware models in QEMU. We have added and interacted with a few educational hardware models in QEMU.