[libvirt] [libvirt-guests] New feature ALWAYS_START

[Marek Lukács]

Hi Martin,

thank you. Definitely I want to propose patch to libvirt-guests, but this
weekend I am taking family holiday, so be patient and wait little bit more.

Regards,

Marek

On Wed, Sep 23, 2015 at 3:12 PM, Martin Kletzander <mkletzan at redhat.com>
wrote:

> On Wed, Sep 23, 2015 at 11:39:29AM +0200, Marek Lukács wrote:
>
>> Hi Martin,
>>
>> thank you for your reply. Please check my comments.
>>
>> On Wed, Sep 23, 2015 at 10:50 AM, Martin Kletzander <mkletzan at redhat.com>
>> wrote:
>>
>> On Tue, Sep 22, 2015 at 07:33:30AM +0200, Marek Lukács wrote:
>>>
>>> Hi,
>>>>
>>>> It will be nice feature to have configuration option
>>>> ALWAYS_START="$uri:$name $uri:name ..." in libvirt-guests
>>>> configuration file.
>>>>
>>>> If ON_BOOT is "start" and if ALWAYS_START is not empty, it iterates
>>>> over the ALWAYS_START and starts guests with same conditions (delays
>>>> etc.) before it starts guests from LISTFILE.
>>>>
>>>>
>>>> To be honest, I don't think that's _exactly_ what you want _just_ from
>>> libvirt itself; let me explain.
>>>
>>> Benefits:
>>>
>>>> - guests are started with delays
>>>>
>>>>
>>> Delays that are done due to the guests are not something we should
>>> handle.  Guests and mainly the applications inside them should handle
>>> this gracefully.  Just delaying the starts is still error-prone.
>>>
>>>
>> I fully understand this and fully agree with you. Production application
>> has to handle this gracefully. But still I see a space for doing it in
>> environments where it is not so necessary to configure application so
>> gracefully, like development or testing environments. Delays are fine for
>> many not production environments, where applications are not in production
>> state.
>>
>>
>> - guests are started after host failure
>>>
>>>>
>>>>
>>> That's what libvirt-guests does already.  And if you want some domains
>>> to be started on every start, there's the 'autostart' parameter for
>>> domains.
>>>
>>>
>> No, libvirt-guests only starts those domains which has been running before
>> "service" libvirt-guests has been stopped. In case of host failure, there
>> is no LISTFILE in filesystem, as it has not been generated by
>> libvirt-guests stop mechanism.
>>
>>
> Oh, so here is what I have misunderstood.  Well, just misread, of
> course libvirt-guests doesn't handle host failures.
>
> But probably I do not understand "script" libvirt-guests correctly and why
>> it is in libvirt. I will be happy, if you will give me more details, why
>> there is this script, even if libvirt has 'autostart' parameter for
>> domains.
>>
>> - For what usage is libvirt-guests designed?
>>
>
> My interpretation is that it is there in order for you to be able to
> shutdown and boot later without losing the machines you were running.
>
> Autostart says that particular domain should be started every time the
> daemon is started.  Basically says that particular guest should be
> running on the host all the time.
>
> - Why it supports delays?
>>
>
> Because most of the time you'll want to resume from managed save and
> you might cause a big load in case you're starting bunch of machines
> because all of those will start loading everything from disk and so
> on.
>
> - Why to have libvirt-guests if there is 'autostart' domain parameter?
>>
>
> It does two different things.  Domain with autostart will be started
> every time daemon (actually not even the computer) is started, but
> libvirt-guests will stop/save domains that are running when the
> computer is being shut down and start/resume them when it is starting
> back.
>
> - For who is libvirt-guests and who should use 'autostart' domain
>> parameter?
>>
>>
> Well, libvirt-guests was added by a guy who was too lazy to clean up
> his machine before rebooting (sorry Jirka, I had to).  But you get the
> picture.  Use it for whatever you like and whatever suits you.  I'm
> not even against adding what you suggested, I just wanted to make sure
> you're not relying on the script for something critical as my
> understanding of it that it is not very error prone.
>
>
>
>> - guests are started in specific order (for example complex
>>>
>>>> environment, when DB should be started before other guest, etc.)
>>>>
>>>>
>>>> Again, same as the first point.  This should be handled gracefully in
>>> the application itself or at least worked around in the guest (not
>>> starting DB-related app before DB is accessible).
>>>
>>> Anyway, if you *really* want this, then the easiest thing to do is
>>> creating a service that starts before libvirt-guests, but after
>>> libvirt, which is just something similar to "local", so it just runs a
>>> script that does:
>>>
>>>  for i in domain_one some_other_domain database_dom
>>>  do
>>>    virsh start "$i"
>>>    sleep 60 # or you can try connecting to make sure it started
>>>  done
>>>
>>> or something similar.  However, you might still propose a simple patch
>>> for the feature you described.
>>>
>>>
>> For me it is no problem to design my own script to handle my needs. I have
>> spent some time googling, if there is already a tool for it. I found only
>> similar questions, so I got feeling, that I am not the only one with
>> similar requirements.
>>
>> Script libvirt-guests in my eyes handles very similar task. It starts
>> domains with delays, it starts domains what has been running at previous
>> stop, but do not handle situation in case of host failure and do not
>> starts
>> domain in specific order. I prefer and I think, that it is better to not
>> create new script no one knows about, but to modify existing one everybody
>> knows about. But again, maybe I do not understand why and for what
>> libvirt-guests is.
>>
>>
> Well, there is no reference purpose and as I said I'm not even against
> adding your option there.  And if we get it into libvirt-guests,
> others might use it.  Would you mind proposing it as a patch?  It
> could be pretty straightforward, I guess.
>
> Have a nice day,
> Martin
>
>
> Anyway, much easier in case of testing and development environments is to
>> set start sequence in /etc/rc.local and forget about script like
>> libvirt-guests, if it does not have features I described.
>>
>>
>> Regards,
>>>
>>>>
>>>> Marek Lukács
>>>>
>>>> --
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>>>> libvir-list at redhat.com
>>>> https://www.redhat.com/mailman/listinfo/libvir-list
>>>>
>>>>
>>>
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