[Engine-devel] Disk BE very small refactoring

Sun Jun 2 08:57:11 UTC 2013

----- Original Message -----
> From: "Omer Frenkel" <ofrenkel at redhat.com>
> To: "Vered Volansky" <vered at redhat.com>
> Cc: "Maor Lipchuk" <mlipchuk at redhat.com>, "Yair Zaslavsky" <yzaslavs at redhat.com>, "Mike Kolesnik"
> <mkolesni at redhat.com>, "Allon Mureinik" <amureini at redhat.com>, "Daniel Erez" <derez at redhat.com>,
> engine-devel at ovirt.org
> Sent: Thursday, May 30, 2013 11:29:40 AM
> Subject: Re: [Engine-devel] Disk BE very small refactoring
> 
> 
> 
> ----- Original Message -----
> > From: "Vered Volansky" <vered at redhat.com>
> > To: "Maor Lipchuk" <mlipchuk at redhat.com>, "Yair Zaslavsky"
> > <yzaslavs at redhat.com>, "Omer Frenkel"
> > <ofrenkel at redhat.com>, "Mike Kolesnik" <mkolesni at redhat.com>, "Allon
> > Mureinik" <amureini at redhat.com>, "Daniel Erez"
> > <derez at redhat.com>
> > Cc: engine-devel at ovirt.org
> > Sent: Tuesday, May 28, 2013 8:40:38 PM
> > Subject: Re: [Engine-devel] Disk BE very small refactoring
> > 
> > I had a problem, didn't see all the replies till now.
> > 
> > I'll add some more info as to why we want to do this like we do.
> > It all started from adding the readOnly property to disks. It should have
> > been handled like plugged is handled, yet plugged is a hack, and if we
> > don't
> > change that now, we'll only keep on adding unreasonable hacks.
> > 
> 
> please explain why you think plugged is a hack? what is wrong with it?
Plugged is a property of a disk-vm.
Were the disk only be able to be plugged to one VM, there would be no problem with that.
But since the disk can be plugged to several VMs, the plugged property should be on the relationship between the two, and not selectively on disk.
Since this is the current case, there's a query that fills in this data in disk under the VM's context, which can be changed when the VM's context is different, which is a hack.

> 
> > So from the start -
> > Disk currently:
> > - Sometimes represents a disk in a vm context and sometimes not.
> > - Holds plugged property, which is only relevant when a disk is in a vm
> > context, which already suggests this is not the natural place for it.
> > - Also holds bootable and interface, which cause limitations of use, but
> > are
> > not so obviously related to the relationship between Disk and Vm as
> > plugged.
> > - Can be shared between several vm's, to some plugged and to some not
> > plugged.
> > - Will soon be optionally RO in one VM and RW in another, which is exactly
> > the same as plugged, and therefore plugged issue should be fixed first.
> > 
> > Every column in that shows a disk in the UI receives a Disk entity, and
> > show
> > its contents, while plugged/unplugged is ignored when not in a VM context.
> > The way things are now, using a VmDevice in the where we need it to show
> > plug
> > status, we'll also have to use it in all other columns, which is irrelevant
> > and just totally not related.
> > So using VmDevice in UI is a no go.
> > 
> > The UI is the main limitation forcing us to use something that extends
> > Disk,
> > and what I described below is the easiest thing to implement in the time
> > restrictions we have without changing the entire system.
> > 
> > I think this answers all the questions not already answered by others.
> > Regarding Maor suggestion - might be a good idea, but not in this scope or
> > time-frame.
> > 
> > If there is any other/unanswered issue or objection to the design change
> > please share.
> > 
> > I appreciate your inputs,
> > Vered
> > 
> 
> sorry i didnt understand why the current disk object isnt good enough,
> you get a disk, some of its properties are valid only in some situations.
> i think its easier to use instead of couple of different objects representing
> the same entity in different situations.
This just doesn't belong there, there's a workaround to make it work when it's indeed needed - by the GetAllDisksByVmIdQuery.
In between the value there means nothing, the fact that we successfully avoid falling into that pitfall doesn't mean it should remain this way.
More importantly, this is not done here, we're adding other properties (currently RO) that face the same handling onless this is resolved now.
This is the best resolution we came up with under the circumstances.

Are you putting you foot down on this?

> 
> > 
> > ----- Original Message -----
> > > From: "Maor Lipchuk" <mlipchuk at redhat.com>
> > > To: "Vered Volansky" <vered at redhat.com>
> > > Cc: engine-devel at ovirt.org
> > > Sent: Tuesday, May 28, 2013 1:49:46 PM
> > > Subject: Re: [Engine-devel] Disk BE very small refactoring
> > > 
> > > I think the main problem is that we abuse the business entity to act as
> > > an O/R mapping class to the DB and also to be used as a business entity
> > > for presentation purposes.
> > > 
> > > I understand how Yair thought isPlugged could be fetched from vmDevice,
> > > this is a confusing design, and it is just one example and more to come.
> > > 
> > > I suggest that if we already thinking of changing the class hierarchy,
> > > we can start by implementing a package for presentation classes for
> > > transient classes such as this instead enforcing complex hierarchy.
> > > 
> > > The query class will fetch all the data from the DB and initialized the
> > > transient class and send it to the client.
> > > I think it could be a good start and will solve many issues we might
> > > encounter in the future.
> > > 
> > > Regards,
> > > Maor
> > > 
> > > On 05/28/2013 11:24 AM, Omer Frenkel wrote:
> > > > 
> > > > 
> > > > ----- Original Message -----
> > > >> From: "Yair Zaslavsky" <yzaslavs at redhat.com>
> > > >> To: "Vered Volansky" <vered at redhat.com>
> > > >> Cc: engine-devel at ovirt.org
> > > >> Sent: Monday, May 27, 2013 6:22:58 PM
> > > >> Subject: Re: [Engine-devel] Disk BE very small refactoring
> > > >>
> > > >> Vered,
> > > >> VmDevice has "isPlugged" field,
> > > >> Why not have somehow in your inheritence (either Disk or a subclass) a
> > > >> field
> > > >> : "VmDevice device"
> > > > 
> > > > disk id is the device id, no need for field to represent the relation.
> > > > the combination of disk-id and a specific entity (vm/template) will get
> > > > you
> > > > the other info
> > > > 
> > > >> and have isPlugged method called "device.isPlugged()" ?
> > > >>
> > > >> Then you can also add the readOnly property which is not represented
> > > >> at
> > > >> VmDevice.
> > > >>
> > > >>
> > > >> Does this sound logical to you?
> > > >>
> > > >> ----- Original Message -----
> > > >>> From: "Vered Volansky" <vered at redhat.com>
> > > >>> To: engine-devel at ovirt.org
> > > >>> Sent: Monday, May 27, 2013 6:18:58 PM
> > > >>> Subject: [Engine-devel] Disk BE very small refactoring
> > > >>>
> > > >>> Hi All,
> > > >>>
> > > >>> Please express your opinion if you have any -
> > > >>>
> > > >>> Currently Disk BE has a plugged property, which should be a property
> > > >>> of
> > > >>> the
> > > >>> relationship between vm(or template) and a disk.
> > > >>> I plan to remove this property from the Disk entity, and add new
> > > >>> entity,
> > > >>> called DeviceDisk.
> > > >>> This should inherit from Disk and contain the vm/template guid and
> > > >>> the
> > > >>> plugged property at first round.
> > > >>> At second round it'll also contain the readOnly property, for RO
> > > >>> disks,
> > > >>> TBD
> > > >>> right after.
> > > >>>
> > > >>> Appreciate any input,
> > > >>> Vered
> > > >>> _______________________________________________
> > > >>> Engine-devel mailing list
> > > >>> Engine-devel at ovirt.org
> > > >>> http://lists.ovirt.org/mailman/listinfo/engine-devel
> > > >>>
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> > > >>
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> > > > 
> > > 
> > > 
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> > 
> 