[Engine-devel] Domain rescan action question

Hopper, Ricky Ricky.Hopper at netapp.com
Wed Aug 1 17:36:09 UTC 2012 


On 8/1/12 10:05 AM, "Dan Yasny" <dyasny at redhat.com> wrote:

>
>
>----- Original Message -----
>> From: "Ricky Hopper" <Ricky.Hopper at netapp.com>
>> To: "Dan Yasny" <dyasny at redhat.com>
>> Cc: engine-devel at ovirt.org, "Itamar Heim" <iheim at redhat.com>, "Andrew
>>Cathrow" <acathrow at redhat.com>
>> Sent: Wednesday, 1 August, 2012 4:56:45 PM
>> Subject: Re: [Engine-devel] Domain rescan action question
>> 
>> 
>> 
>> On 8/1/12 9:42 AM, "Dan Yasny" <dyasny at redhat.com> wrote:
>> 
>> >
>> >
>> >----- Original Message -----
>> >> From: "Ricky Hopper" <Ricky.Hopper at netapp.com>
>> >> To: "Dan Yasny" <dyasny at redhat.com>, "Andrew Cathrow"
>> >><acathrow at redhat.com>
>> >> Cc: engine-devel at ovirt.org, "Itamar Heim" <iheim at redhat.com>,
>> >> "Ricky
>> >>Hopper" <Ricky.Hopper at netapp.com>
>> >> Sent: Wednesday, 1 August, 2012 4:34:53 PM
>> >> Subject: Re: [Engine-devel] Domain rescan action question
>> >> 
>> >> 
>> >> 
>> >> On 8/1/12 5:59 AM, "Dan Yasny" <dyasny at redhat.com> wrote:
>> >> 
>> >> >
>> >> >
>> >> >----- Original Message -----
>> >> >> From: "Andrew Cathrow" <acathrow at redhat.com>
>> >> >> To: "Itamar Heim" <iheim at redhat.com>, "Dan Yasny"
>> >> >> <dyasny at redhat.com>,
>> >> >>"Ricky Hopper" <Ricky.Hopper at netapp.com>
>> >> >> Cc: engine-devel at ovirt.org
>> >> >> Sent: Wednesday, 1 August, 2012 12:24:42 AM
>> >> >> Subject: Re: [Engine-devel] Domain rescan action question
>> >> >> 
>> >> >> 
>> >> >> 
>> >> >> ----- Original Message -----
>> >> >> > From: "Itamar Heim" <iheim at redhat.com>
>> >> >> > To: "Ricky Hopper" <Ricky.Hopper at netapp.com>
>> >> >> > Cc: engine-devel at ovirt.org
>> >> >> > Sent: Tuesday, July 31, 2012 4:44:34 PM
>> >> >> > Subject: Re: [Engine-devel] Domain rescan action question
>> >> >> > 
>> >> >> > On 07/31/2012 11:30 PM, Hopper, Ricky wrote:
>> >> >> > > Hey all,
>> >> >> > >
>> >> >> > > As I'm making progress with the domain rescan
>> >> >> > > functionality,
>> >> >> > > I've
>> >> >> > > realized that I'm unsure what to do with any disks that are
>> >> >> > > detected on
>> >> >> > > the domain. Should I add them back into the database to be
>> >> >> > > listed
>> >> >> > > as
>> >> >> > > floating disks, or should I just return a list of disk
>> >> >> > > images
>> >> >> > > to
>> >> >> > > be
>> >> >> > > attached to whatever the caller of the query needs?
>> >> >> > >
>> >> >> > > - Ricky
>> >> >> > 
>> >> >> > i'm not sure they should be added automatically.
>> >> >> > I think a dialog[1] showing orphan disks/images on the
>> >> >> > storage
>> >> >> > domain
>> >> >> > for user to choose which to import as 'floating' disks would
>> >> >> > be
>> >> >> > better
>> >> >> > than auto importing them.
>> >> >> > 
>> >> >> > there is also the reverse of flagging existing disks as
>> >> >> > 'missing'
>> >> >> > in
>> >> >> > storage?
>> >> >> > 
>> >> >> 
>> >> >> Perhaps we should start a feature page to discuss and better
>> >> >> scope
>> >> >> it.
>> >> >> There is a feature page that we could expand, it doesn't
>> >> >> discuss
>> >> >> the
>> >> >> notion of importing those disks which is certainly something we
>> >> >> need
>> >> >> to address.
>> >> >> 
>> >> >> 
>> >> >> http://wiki.ovirt.org/wiki/Features/Orphaned_Images
>> >> >
>> >> >The original idea was to scan the storage domains and compare the
>> >> >images
>> >> >lists to the database, thus getting a list of images no longer
>> >> >relevant
>> >> >and scrubbing the storage. This will actually be addressed
>> >> >properly
>> >> >in
>> >> >the future (Ayal can elaborate on that) but for now this is
>> >> >needed
>> >> >at
>> >> >least for that use case.
>> >> >
>> >> >
>> >> >As I understand, the conversation here is about trying to take an
>> >> >already
>> >> >populated SD (from another setup I suppose), scanning it and
>> >> >putting
>> >> >it
>> >> >into RHEV?
>> >> 
>> >> As I understood it, the purpose of this functionality wasn't to
>> >> find
>> >> images which should be removed from storage, but to find images on
>> >> the
>> >> domain that oVirt was unaware of and importing them for use (for
>> >> instance,
>> >> if a disk was created outside of oVirt on the domain). If one of
>> >> the
>> >> use
>> >> cases for this feature is also the orphaned images mentioned on
>> >> the
>> >> feature page, that may expand the functionality into a separate
>> >> domain
>> >> scrub and storage import, both of which would call the rescan
>> >> (meaning the
>> >> rescan would not actually add to the database, but instead return
>> >> a
>> >> list
>> >> of "orphaned" disk images).
>> >> 
>> >> Another solution would be to import all disk images into the
>> >> database
>> >> either way, and let the user delete any orphaned images from the
>> >> GUI.
>> >
>> >I think are nice to have, but the problem with the scanning is that
>> >if
>> >we're not scanning a master domain or an export domain, all we will
>> >see
>> >is a bunch of images with no context or even hints as to where they
>> >belong. The data that makes it all usable is in the engine database
>> >and
>> >in the ovf files on the master domain.
>> >
>> >This is why I stopped at the orphaned images part of the feature -
>> >because there it's feasible, I would rely on the engine database for
>> >image ID comparisons.
>> >
>> >If we present a user with a list of nameless disks, I doubt it will
>> >be of
>> >any use.
>> 
>> The way this would work is by comparing a list of disk images from
>> vdsm
>> and from oVirt's database, finding the ones vdsm returns that oVirt
>> doesn't have, and then either adding or returning those images. So
>> oVirt's
>> db will be used in the comparison.
>
>This will work only when scanning storage domains already attached and in
>use by the current oVirt setup. What I am talking about is what will
>happen if a LUN that used to be a SD in another oVirt setup is discovered
>and scanned, with no engine db to compare with. If we don't consider such
>a use case, life is definitely quite easy, and we're basically within the
>scope of the orphaned images feature

This use case should definitely be considered, maybe have a separate case
where the rescan would return all "compatible" disks (i.e. disks that
aren't just partial snapshots and the like) if the domain has not yet been
mounted. Essentially, it would run the same comparison, but compare
against an empty list rather than a list of disks. There's no way it's as
simple as that (I'm unsure of the methods oVirt uses to mount a domain),
but it's a good starting point.
>
>> 
>> As far as presenting the user with nameless disks, that's a point I
>> hadn't
>> considered; we could generate some sort of placeholder metadata upon
>> addition to show the user that these are new/orphaned disks that were
>> found on the storage domain. Is it safe to assume that the disks
>> discovered by this feature won't be attached to anything?
>
>The oVirt paradigm says "if it isn't in the engine db, it's not ours", so
>any LV or image we discover that is missing from the DB or the snapshot
>chain of the image in the DB, is nameless, and orphaned.
>
>Such an image on a current SD, belonging to a working oVirt setup is
>definitely an orphaned image. Attaching these to VMs is usually also
>useless, because they are more often than not discarded snapshots that
>didn't get discarded cleanly for some reason.
>
>
>Now, if we want to make this usable, we might want to actually check the
>qcow2 metadata of the image to see whether it's a mid-chain snapshot (and
>if so it's probably just a candidate for cleanup), or a standalone qcow2
>or raw image, and then we can move on with the virt-* tools, to find out
>the image size and the filesystems it contains. This will at least
>provide the user with some usable information about the detected image.
>If we're talking about scanning an SD that doesn't presently belong to
>the current oVirt setup, then this is even more relevant, because all of
>the images will have no VM-related context.

We're currently working on having disks created outside of the oVirt
environment, so not all orphaned disks on the existing storage domain will
be artifacts of supposedly-deleted data. For our use case, disk images
created by us will be able to be imported into oVirt and attached to a VM
created through the engine. Because of this, saying "if it isn't in the
engine db, it's not ours" wouldn't necessarily be true.

When you talk about checking the metadata, does either oVirt or vdsm have
a simple way to do this? A query of some sort would be ideal for this, as
it could be run for each image as a qualifier for import.

Also, as far as writing the functionality itself, I'm gathering that it
should be structured as a query to return these orphaned images, which can
then be acted upon/added to the database through a separate command after
checking the validity of each image?
>
>> > 
>> >
>> >> >
>> >> >> 
>> >> >> > 
>> >> >> > [1] or a subtab on the storage domain.
>> >> >> > 
>> >> >> > _______________________________________________
>> >> >> > Engine-devel mailing list
>> >> >> > Engine-devel at ovirt.org
>> >> >> > http://lists.ovirt.org/mailman/listinfo/engine-devel
>> >> >> > 
>> >> >> 
>> >> >
>> >> >--
>> >> >
>> >> >
>> >> >
>> >> >Regards,
>> >> >
>> >> >Dan Yasny
>> >> >Red Hat Israel
>> >> >+972 9769 2280
>> >> 
>> >> 
>> >
>> >--
>> >
>> >
>> >
>> >Regards,
>> >
>> >Dan Yasny
>> >Red Hat Israel
>> >+972 9769 2280
>> 
>> 
>
>-- 
>
>
>
>Regards, 
>
>Dan Yasny 
>Red Hat Israel 
>+972 9769 2280

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