Re: [Xen-devel] [Qemu-devel] [PATCH v7 2/6] Introduce "save-devices-state"

[Eric Blake <eblake@xxxxxxxxxx>]

On 03/16/2012 06:13 AM, Stefano Stabellini wrote:
> - add an "is_ram" flag to SaveStateEntry;
> 
> - register_savevm_live sets is_ram for live_savevm devices;
> 
> - introduce a "save-devices-state" QAPI command that can be used to save
> the state of all devices, but not the RAM or the block devices of the
> VM.
> 

> +QEMU has code to load/save the state of the guest that it is running.
> +These are two complementary operations.  Saving the state just does
> +that, saves the state for each device that the guest is running.
> +
> +These operations are normally used with migration (see migration.txt),
> +however it is also possible to save the state of all devices to file,
> +without saving the RAM or the block devices of the VM.
> +
> +This operation is called "save-devices-state" (see QMP/qmp-commands.txt)

Is there a complimentary load-devices-state?

Just to make sure I'm clear, there are three things to save before you
have a complete picture of a running VM:

disk state (can be saved with 'savevm' to internal qcow2 snapshots, or
with 'transaction' and 'blockdev-snapshot-sync' by creating external
snapshots, or by 'migrate' to a file)

RAM state (can be saved with 'savevm' to internal qcow2 snapshot, or
with 'migrate' to a file; it is also possible to start qemu with a
command line parameter telling which backing file tracks RAM state)

device state (can be saved with 'savevm' to internal qcow2 snapshot, or
with 'migrate' to a file, and now with 'save-devices-state')

That is, 'migrate' does all three at once to an external file, 'savevm'
does all three at once to an internal qcow2 snapshot, and you are now
making it possible to do any one of the three independently to an
external file while the VM continues to run.

Is this something that libvirt should be exposing in the near future?

> +SQMP
> +save-devices-state
> +-------
> +
> +Save the state of all devices to file. The RAM and the block devices
> +of the VM are not saved by this command.
> +
> +Arguments:
> +
> +- "filename": the file to save the state of the devices to as binary
> +data. See save-devices-state.txt for a description of the binary format.
> +
> +Example:
> +
> +-> { "execute": "save-devices-state", "arguments": { "filename": "/tmp/save" 
> } }

Can we at least reserve something for future extension to add things
like 'fd:name' to refer to a named fd received earlier via 'getfd'?  You
could do that by documenting up front that if "filename" does not start
with "/", then any pattern of letters followed by a ':' as the initial
pattern of the filename is an error (and you avoid the error by using
./foo:... or an absolute path).

-- 
Eric Blake   eblake@xxxxxxxxxx    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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