[Engine-devel] ovirt core MOM

[Itamar Heim]

On 01/23/2012 11:01 PM, Ayal Baron wrote:
>
>
> ----- Original Message -----
>>
>>
>> ----- Original Message -----
>>> From: "Ayal Baron"<abaron at redhat.com>
>>> To: "Itamar Heim"<iheim at redhat.com>
>>> Cc: engine-devel at ovirt.org, "Miki Kenneth"<mkenneth at redhat.com>
>>> Sent: Sunday, January 22, 2012 11:19:03 AM
>>> Subject: Re: [Engine-devel] ovirt core MOM
>>>
>>>
>>>
>>> ----- Original Message -----
>>>> On 01/20/2012 11:42 PM, Miki Kenneth wrote:
>>>>>
>>>>>
>>>>> ----- Original Message -----
>>>>>> From: "Itamar Heim"<iheim at redhat.com>
>>>>>> To: "Ayal Baron"<abaron at redhat.com>
>>>>>> Cc: engine-devel at ovirt.org
>>>>>> Sent: Friday, January 20, 2012 2:12:27 AM
>>>>>> Subject: Re: [Engine-devel] ovirt core MOM
>>>>>>
>>>>>> On 01/19/2012 11:58 AM, Ayal Baron wrote:
>>>>>>>
>>>>>>>
>>>>>>> ----- Original Message -----
>>>>>>>> On 01/18/2012 05:53 PM, Livnat Peer wrote:
>>>>>>>>> Hi All,
>>>>>>>>>
>>>>>>>>> This is what we've discussed in the meeting today:
>>>>>>>>>
>>>>>>>>> Multiple storage domain:
>>>>>>>>> - Should have a single generic verb for removing a disk.
>>>>>>>>> - We block removing the last template disk - template is
>>>>>>>>> immutable.
>>>>>>>>
>>>>>>>> but it will be deleted when deleting the template, right?
>>>>>>>
>>>>>>> Of course.
>>>>>>> The point is that the template is an immutable object and
>>>>>>> should
>>>>>>> not change (until we support editing a template at which
>>>>>>> point
>>>>>>> the
>>>>>>> user would have to change the template to edit mode before
>>>>>>> being
>>>>>>> able to make such changes and maybe also be able to run it
>>>>>>> and
>>>>>>> make changes internally?).
>>>>>>
>>>>>> When i hear "edit a template" i don't expect replacing the
>>>>>> files.
>>>>>> I expect a new edition of disks appearing as a new version of
>>>>>> the
>>>>>> template. but they don't have to derive from same original
>>>>>> template.
>>>>>> say i want to create a "Fedora 16 template", then update it
>>>>>> every
>>>>>> month
>>>>>> with latest "yum update".
>>>>>> it doesn't matter if i use a VM from same template or just
>>>>>> create
>>>>>> a
>>>>>> new one.
>>>>>> then specify it is V2 of the "Fedora 16 template".
>>>>>> when someone creates a VM from this template, default version
>>>>>> would
>>>>>> be
>>>>>> latest (but we can let them choose specific older versions as
>>>>>> well)
>>>>> +1. Nicely put.
>>>>> And just to add another common use case is the pool usage.
>>>>> When we creating stateless VM pool from the template,
>>>>> it would be nice to be able to update the template to V2,
>>>>> and have all the newly created VMs dynamically based to the new
>>>>> template.
>>>>
>>>> that is indeed where i was going with it as well, but not as
>>>> trivial,
>>>> since need to wait for VMs to stop and return to pool and create
>>>> new
>>>> ones and remove old ones.
>>>> also, creating new ones may involve an admin action of first boot
>>>> +
>>>> take
>>>> of first snapshot
>>>>
>>>> (hence i stopped the previous description before this part, but
>>>> since
>>>> you opened the door...)
>>>
>>> Yes, but this all goes to template versioning (which is great and
>>> we
>>> need).
>>> For the user though, creating a new template version like you
>>> described would be a long and wasteful process, and is not what I'm
>>> talking about.
>>>
>>> Unless we support nested templates (second template would be a
>>> snapshot over the first one), then we're likely to require way too
>>> much space and creation process would be too slow (having to copy
>>> over all the bits).
>>> I think the pool example is an excellent example where I would not
>>> want to have 2 copies of the template where the only difference
>>> between them is a set of security patches I've applied to the new
>>> template.
>> Not sure I understand how you do that while vms are still running on
>> the original template?
>
> They either:
> 1. wouldn't be (if changes are in place)
> 2. if we support template over template (from snapshot) then no issue at all.
>     Once all VMs stop running on previous template we can live merge the 2.
>
>>>
>>> So the 2 options are for what I'm suggesting are:
>>> 1. decommission the old template by making in place changes
>>> 2. support template snapshots
>> Not sure how this will work and what use case it serves?
>
> number 1: changing the template for stateless pools.
> number 2: for anything you want including template versioning.
> Template versioning should have 2 flavours:
> 1. my golden image is outdated and I would like to decommission it and replace with a new one created from scratch (i.e. same name, new VMs would be derived from new template, no data dedup).
> 2. my golden image is outdated and I would like to update it internally - create a VM from it, make the changes, seal this VM as the new version of the template (not using the process we have today which copies all the data, just change it to be immutable).
>
> The latter requires supporting trees.

use case wise, #1 is easier, and covers both use cases - it only varies 
in amount of IO/Space, so when someone tackles this implementation wise, 
I'd vote for doing #1 first.