libvirt users - Oct 2020 - Re: Determining when a guest booted / how long it's been running

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On 10/1/20 3:42 AM, Digimer wrote:
> Hi all,
> 
>  ? Is there a way to tell when a tool made a change to guest (ie: used 
> virt-manager to make a change)? Following, is there a way to check to 
> see if there are changes queued to take effect when the guest next reboots?
> 
You can listen for events. For changes to inactive XML you will get 
VIR_DOMAIN_EVENT_DEFINED+VIR_DOMAIN_EVENT_DEFINED_UPDATED lifecycle 
event (+reason) and for changes to live XML (like hotplug and hotunplug) 
you'll get DEVICE_ADDED or DEVICE_REMOVED.

But there is no API/virsh command that you could call to get the 
timestamp of last modification. You have to listen for events.

And the second question - you can dump live and inactive XML and see if 
there is any difference (modulo runtime configuration from live XML). 
The live XML should be strictly bigger than inactive XML, meaning live 
XML should be inactive XML + runtime info. But again, no API there, 
because changes to inactive XML can be done any time - libvirt keeps 
live and inactive XMLs separately.
>  ? If either of the above are not possible, is there a way to see when a 
> guest last booted or how long a guest has been running?
Again, if listening to evens - libvirt emits one when a guest is 
started. But I don't think we store a timestamp of start anywhere nor 
expose it through an API. Maybe there is some indirect way?

Michal

daggs <daggs@gmx.com> writes:
> I'd assume that saying vm running you mean that the os is up and
> running too.  I have similar need, I was able to get something as such
> to work using virsh console when the guest was a linux with serial
> console support enabled.  I wasn't able to get this to work in a
> script as I was never able to terminate the console seesion as virsh
> console wasn't able to send the termination combination properly.
I wouldn't have assumed this meaning, but running off the tangent
anyway: I think installing a guest agent could help solving this
problem.  However, if you want something extremely lightweight, the
following (a custom one-line guest agent if you wish) worked for me:

1. Add a channel with a descriptive name to the guest config:

   <channel type="unix">
     <target type="virtio" name="startup_signal"/>
   </channel>

2. After starting the domain, query the assigned socket path:

   virsh dumpxml $domain | xmllint --nonet --xpath \
   "string(/domain/devices/channel[@type='unix' and
target/@name='startup_signal']/source/@path)" -)

3. Then wait for the signal (and optionally acknowledge it):

   socat UNIX-CONNECT:"$socket_path" \
      SYSTEM:'read msg id && [ $msg = ready ] && echo ack
$id'

This will wait until you send the signal from the chosen point of the
guest bootup procedure via something like:

port=/dev/virtio-ports/startup_signal
msg=foobar
reply="$(echo "ready $msg" | socat STDIO "$port")"
[ "($reply)" = "(ack $msg)" ] # for checking proper
acknowledgment

I folded this into a oneshot systemd service on Linux, but the
implementation should be similarly simple under any OS.
-- 
Feri

On 2020-10-01 1:21 a.m., Michal Prívozník wrote:
> On 10/1/20 3:42 AM, Digimer wrote:
>> Hi all,
>>
>>    Is there a way to tell when a tool made a change to guest (ie: used
>> virt-manager to make a change)? Following, is there a way to check to
>> see if there are changes queued to take effect when the guest next
>> reboots?
>>
> 
> You can listen for events. For changes to inactive XML you will get
> VIR_DOMAIN_EVENT_DEFINED+VIR_DOMAIN_EVENT_DEFINED_UPDATED lifecycle
> event (+reason) and for changes to live XML (like hotplug and hotunplug)
> you'll get DEVICE_ADDED or DEVICE_REMOVED.
> 
> But there is no API/virsh command that you could call to get the
> timestamp of last modification. You have to listen for events.
> 
> And the second question - you can dump live and inactive XML and see if
> there is any difference (modulo runtime configuration from live XML).
> The live XML should be strictly bigger than inactive XML, meaning live
> XML should be inactive XML + runtime info. But again, no API there,
> because changes to inactive XML can be done any time - libvirt keeps
> live and inactive XMLs separately.
> 
>>    If either of the above are not possible, is there a way to see when
>> a guest last booted or how long a guest has been running?
> 
> Again, if listening to evens - libvirt emits one when a guest is
> started. But I don't think we store a timestamp of start anywhere nor
> expose it through an API. Maybe there is some indirect way?
> 
> Michal
Thanks, Michal.

Now I know not to keep looking. I do infer the boot time (explained in
my reply to daggs and feri. In brief, when we notice a server is
running, that had last been seen off, we use the pid to get the runtime
in seconds and subtract that from the current time to get a boot time.

-- 
Digimer
Papers and Projects: https://alteeve.com/w/
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