[libvirt] Disk snapshot mode proposal: patch for storing the snapshot mode from .vmx to .xml

[computernews at rambler.ru]

Hello,

So what I see from the discussion is that I have to reimplement my patch 
one way or another (attribute / sub-element). So I am eager to do that 
as soon as the choice is made (unless you decide otherwise and want 
someone else to implement that).

Now that you have explained the bigger picture with more details (other 
hypervisors but the ESX)  let me summarize an upcoming changes into a 
specification:
Value in .vmx file
	representation in <domain> XML" as attributes
	representation in <domain> XML" as sub-items 	Description
scsi0:0.mode = "persistent" 	<disk ... snapshot='yes' persistent='yes'>
	
	Default. Snapshots are in use, changes survive power cycles
scsi0:0.mode = "independent-persistent" 	<disk ... snapshot='no' 
persistent='yes'> 	<independent/> 	No snapshot logic is applicable, 
changes remain
scsi0:0.mode = "independent-nonpersistent" 	<disk ... snapshot='no' 
persistent='no'> 	<independent/> <nonpersistent/> 	No snapshots, changes 
lost after domain power cycles
scsi0:0.mode = "undoable" 	<disk ... snapshot='no' persistent='yes'> 
<independent/> 	for backwards compatibility
scsi0:0.mode = "nonpersistent" 	<disk ... snapshot='no' 
persistent='no'> 	<independent/> <nonpersistent/> 	for backwards 
compatibility


Also the reverse conversion has to be implemented (<domain> XML -> vmx 
file).

Obviously the appropriate <domain> XML format description changes has to 
be made here: http://libvirt.org/formatdomain.html#elementsDisks. I can 
do that as well.

Best regards
Oleh Paliy


> On 07/09/2011 09:52 AM, Matthias Bolte wrote:
>> Anyway, you decided to add an snapshot_mode attribute to the disk
>> element and exposed the VMX values there. I'm not sure that this is a
>> good idea as scsi0:0.mode affects two aspects.
>>
>> scsi0:0.mode can basically have three different modes
>>
>> - persistent, the default, a disk with this mode will take part in
>> snapshots and changes to the disk's content persist domain power
>> cycles and snapshot restoring.
>>
>> - independent-persistent, a disk with this mode will not take part in
>> snapshots, but changes to the disk's content persist domain power
>> cycles and snapshot restoring.
>>
>> - independent-nonpersistent, a disk with this mode will be not take
>> part in snapshots and changes to the disk's content don't persist
>> domain power cycles and snapshot restoring. This is realized by
>> writing all changes into an additional .vmdk instead of the original
>> .vmdk. This additional .vmdk is automatically deleted on domain power
>> cycles and snapshot restoring.
>>
>> There are two additional but outdated modes undoable and nonpersistent
>> that aren't supported anymore.
>>
>> So the two aspects scsi0:0.mode affects is snapshot and the
>> persistence of changes. I think it makes more sense to use two
>> attributes for the disk element to expose this.
>>
>> <disk ... snapshot='yes|no' persistent='yes|no'>
>>
>> - snapshot=yes persistent=yes maps to scsi0:0.mode=persistent
>>
>> - snapshot=yes persistent=no is unsupported for ESX
>>
>> - snapshot=no persistent=yes maps to scsi0:0.mode=independent-persistent
>>
>> - snapshot=no persistent=no maps to scsi0:0.mode=independent-nonpersistent
> Hmm, this indeed seems like it might be reasonable to represent both
> aspects in XML.  See also
> https://www.redhat.com/archives/libvir-list/2011-May/msg00315.html.
>
> At stake is whether a disk has a snapshot taken by default, and whether
> a disk is treated as temporary for the life of the domain (qemu has a
> -snapshot command line option that treats all disks as temporary, but
> with better per-volume snapshot abilities, libvirt could certainly offer
> the same fine-tuning of per-disk as esx appears to offer).
>
> So yes, I'll need to fold something like this into my v2 proposal for
> snapshot handling.
>
>> The more important question is: Is this an general concept or is it
>> too ESX specific?
> It's sounding generic enough that it will be worth getting it right in
> the XML.
>
>> Eric is currently discussing/designing an extension to libvirt's
>> snapshot/checkpoint capabilities. At the moment libvirt supports
>> checkpointing a complete domain including RAM image and all storage
>> volumes, but it doesn't support snapshotting of single storage volumes
>> or a subset of the storage volumes of a domain.
>>
>> https://www.redhat.com/archives/libvir-list/2011-June/msg00761.html
>>
>> Eric suggest to extend the<domainsnapshot>  and virStorageVol* APIs to
>> allow to include only a subset of the domain's storage volumes in a
>> checkpoint. This approach allows to specifiy for each checkpoint which
>> storage volumes to include. ESX allows something similar with the
>> independent modes, but you cannot define this on a per snapshot basis,
>> but have to decided this before. Eric's approach is more flexible but
>> doesn't work for ESX. I wonder if we could add the snapshot
>> attribute/subelement to the disk element. This allows to set the
>> independent mode for ESX and allows to define a preset for other
>> hypervisors like QEMU that will support Eric's more flexible approach.
>>
>> So when you don't explicitly define which disk to include in a
>> checkpoint in the<domainsnapshot>  XML then the snapshot setting from
>> the the<domain>  XML apply. If there are no presets for snapshot it
>> defaults to yes. For QEMU you could override the snapshot setting from
>> the<domain>  XML in the<domainsnapshot>  XML, for ESX you either don't
>> specify this in the<domainsnapshot>  XML or have to match the settings
>> from the<domain>  XML due to the way ESX works.
> Yes, having a per-disk default in the<domain>  XML (applicable to both
> qemu and ESX), as well as a per-disk override in the<domainsnapshot>
> (here, qemu can take advantage, but ESX would have to fail if the
> override is not identical to the domain defaults). does make sense.
>
>> The persistence setting might be more ESX specific, but I think
>> libvirt could realize this for QEMU too, when the domain is using
>> qcow2 images with a base image. In that case libvirt could clear the
>> qcow2 image when the domain is restarted to realize persistent=no. I
>> might be incorrect here as I'm not the QEMU expert here.
> You're exactly right - qemu can implement per-disk persistent=no by
> doing a qcow2 wrapper around just the disks that should be reverted when
> the VM stops running.  There might be some interactions with migration
> to worry about, though.
>
>> On a second thought we might want to use negative word so we don't add
>> subelements for the defaults, for example
>>
>>      <disk type='file' device='disk'>
>>        <source file='[datastore] test1/test1.vmdk'/>
>>        <target dev='sda' bus='scsi'/>
>>        <independent/>
>>        <nonpersistent/>
>>        <address type='drive' controller='0' bus='0' unit='0'/>
>>      </disk>
> Now we're doing a bit of bike-shedding - I think there's definitely
> consensus that this has to be in the XML somewhere, but whether as an
> attribute or as a sub-element still remains to be decided.
>

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