[Engine-devel] [vdsm] RFC: New Storage API

Ayal Baron abaron at redhat.com
Mon Jan 14 21:34:05 UTC 2013 


----- Original Message -----
> 
> 
> ----- Original Message -----
> > From: "Itamar Heim" <iheim at redhat.com>
> > To: "Saggi Mizrahi" <smizrahi at redhat.com>
> > Cc: "VDSM Project Development" <vdsm-devel at lists.fedorahosted.org>,
> > "engine-devel" <engine-devel at ovirt.org>
> > Sent: Monday, January 14, 2013 6:18:13 AM
> > Subject: Re: [vdsm] RFC: New Storage API
> > 
> > On 12/04/2012 11:52 PM, Saggi Mizrahi wrote:
> > > I've been throwing a lot of bits out about the new storage API
> > > and
> > > I think it's time to talk a bit.
> > > I will purposefully try and keep implementation details away and
> > > concentrate about how the API looks and how you use it.
> > >
> > > First major change is in terminology, there is no long a storage
> > > domain but a storage repository.
> > > This change is done because so many things are already called
> > > domain in the system and this will make things less confusing for
> > > new-commers with a libvirt background.
> > >
> > > One other changes is that repositories no longer have a UUID.
> > > The UUID was only used in the pool members manifest and is no
> > > longer needed.
> > >
> > >
> > > connectStorageRepository(repoId, repoFormat,
> > > connectionParameters={}):
> > > repoId - is a transient name that will be used to refer to the
> > > connected domain, it is not persisted and doesn't have to be the
> > > same across the cluster.
> > > repoFormat - Similar to what used to be type (eg. localfs-1.0,
> > > nfs-3.4, clvm-1.2).
> > > connectionParameters - This is format specific and will used to
> > > tell VDSM how to connect to the repo.
> > >
> > > disconnectStorageRepository(self, repoId):
> > >
> > >
> > > In the new API there are only images, some images are mutable and
> > > some are not.
> > > mutable images are also called VirtualDisks
> > > immutable images are also called Snapshots
> > >
> > > There are no explicit templates, you can create as many images as
> > > you want from any snapshot.
> > >
> > > There are 4 major image operations:
> > >
> > >
> > > createVirtualDisk(targetRepoId, size, baseSnapshotId=None,
> > >                    userData={}, options={}):
> > >
> > > targetRepoId - ID of a connected repo where the disk will be
> > > created
> > > size - The size of the image you wish to create
> > > baseSnapshotId - the ID of the snapshot you want the base the new
> > > virtual disk on
> > > userData - optional data that will be attached to the new VD,
> > > could
> > > be anything that the user desires.
> > > options - options to modify VDSMs default behavior
> > >
> > > returns the id of the new VD
> > >
> > > createSnapshot(targetRepoId, baseVirtualDiskId,
> > >                 userData={}, options={}):
> > > targetRepoId - The ID of a connected repo where the new sanpshot
> > > will be created and the original image exists as well.
> > > size - The size of the image you wish to create
> > > baseVirtualDisk - the ID of a mutable image (Virtual Disk) you
> > > want
> > > to snapshot
> > > userData - optional data that will be attached to the new
> > > Snapshot,
> > > could be anything that the user desires.
> > > options - options to modify VDSMs default behavior
> > >
> > > returns the id of the new Snapshot
> > >
> > > copyImage(targetRepoId, imageId, baseImageId=None, userData={},
> > > options={})
> > > targetRepoId - The ID of a connected repo where the new image
> > > will
> > > be created
> > > imageId - The image you wish to copy
> > > baseImageId - if specified, the new image will contain only the
> > > diff between image and Id.
> > >                If None the new image will contain all the bits of
> > >                image Id. This can be used to copy partial parts
> > >                of
> > >                images for export.
> > > userData - optional data that will be attached to the new image,
> > > could be anything that the user desires.
> > > options - options to modify VDSMs default behavior
> > >
> > > return the Id of the new image. In case of copying an immutable
> > > image the ID will be identical to the original image as they
> > > contain the same data. However the user should not assume that
> > > and
> > > always use the value returned from the method.
> > >
> > > removeImage(repositoryId, imageId, options={}):
> > > repositoryId - The ID of a connected repo where the image to
> > > delete
> > > resides
> > > imageId - The id of the image you wish to delete.
> > >
> > >
> > > ----
> > > getImageStatus(repositoryId, imageId)
> > > repositoryId - The ID of a connected repo where the image to
> > > check
> > > resides
> > > imageId - The id of the image you wish to check.
> > >
> > > All operations return once the operations has been committed to
> > > disk NOT when the operation actually completes.
> > > This is done so that:
> > > - operation come to a stable state as quickly as possible.
> > > - In case where there is an SDM, only small portion of the
> > > operation actually needs to be performed on the SDM host.
> > > - No matter how many times the operation fails and on how many
> > > hosts, you can always resume the operation and choose when to do
> > > it.
> > > - You can stop an operation at any time and remove the resulting
> > > object making a distinction between "stop because the host is
> > > overloaded" to "I don't want that image"
> > >
> > > This means that after calling any operation that creates a new
> > > image the user must then call getImageStatus() to check what is
> > > the status of the image.
> > > The status of the image can be either optimized, degraded, or
> > > broken.
> > > "Optimized" means that the image is available and you can run VMs
> > > of it.
> > > "Degraded" means that the image is available and will run VMs but
> > > it might be a better way VDSM can represent the underlying data.
> > > "Broken" means that the image can't be used at the moment,
> > > probably
> > > because not all the data has been set up on the volume.
> > >
> > > Apart from that VDSM will also return the last persisted status
> > > information which will conatin
> > > hostID - the last host to try and optimize of fix the image
> > > stage - X/Y (eg. 1/10) the last persisted stage of the fix.
> > > percent_complete - -1 or 0-100, the last persisted completion
> > > percentage of the aforementioned stage. -1 means that no progress
> > > is available for that operation.
> > > last_error - This will only be filled if the operation failed
> > > because of something other then IO or a VDSM crash for obvious
> > > reasons.
> > >               It will usually be set if the task was manually
> > >               stopped
> > >
> > > The user can either be satisfied with that information or as the
> > > host specified in host ID if it is still working on that image by
> > > checking it's running tasks.
> > >
> > > checkStorageRepository(self, repositoryId, options={}):
> > > A method to go over a storage repository and scan for any
> > > existing
> > > problems. This includes degraded\broken images and deleted images
> > > that have no yet been physically deleted\merged.
> > > It returns a list of Fix objects.
> > > Fix objects come in 4 types:
> > > clean - cleans data, run them to get more space.
> > > optimize - run them to optimize a degraded image
> > > merge - Merges two images together. Doing this sometimes
> > >          makes more images ready optimizing or cleaning.
> > >          The reason it is different from optimize is that
> > >          unmerged images are considered optimized.
> > > mend - mends a broken image
> > >
> > > The user can read these types and prioritize fixes. Fixes also
> > > contain opaque FIX data and they should be sent as received to
> > > fixStorageRepository(self, repositoryId, fix, options={}):
> > >
> > > That will start a fix operation.
> > >
> > >
> > > All major operations automatically start the appropriate "Fix" to
> > > bring the created object to an optimize\degraded state (the one
> > > that is quicker) unless one of the options is
> > > AutoFix=False. This is only useful for repos that might not be
> > > able
> > > to create volumes on all hosts (SDM) but would like to have the
> > > actual IO distributed in the cluster.
> > >
> > > Other common options is the strategy option:
> > > It has currently 2 possible values
> > > space and performance - In case VDSM has 2 ways of completing the
> > > same operation it will tell it to value one over the other. For
> > > example, whether to copy all the data or just create a qcow based
> > > of a snapshot.
> > > The default is space.
> > >
> > > You might have also noticed that it is never explicitly specified
> > > where to look for existing images. This is done purposefully,
> > > VDSM
> > > will always look in all connected repositories for existing
> > > objects.
> > > For very large setups this might be problematic. To mitigate the
> > > problem you have these options:
> > > participatingRepositories=[repoId, ...] which tell VDSM to narrow
> > > the search to just these repositories
> > > and
> > > imageHints={imgId: repoId} which will force VDSM to look for
> > > those
> > > image ID just in those repositories and fail if it doesn't find
> > > them there.
> > > _______________________________________________
> > > vdsm-devel mailing list
> > > vdsm-devel at lists.fedorahosted.org
> > > https://lists.fedorahosted.org/mailman/listinfo/vdsm-devel
> > >
> > 
> > you are using VirtualDisk and Snapshot for mutable and immutable.
> > the terminology of "image" for immutable and "volume" for mutable
> > seems
> > to be the one used by ec2/openstack/etc. - any thoughts on using
> > similar
> > terminology?
> > (though i think they also differ in forcing all images to be in an
> > image
> > repo, and all volumes in a volume repo, while we allow to mix them
> > in
> > same repo).
> > 
> We have volumes internally though, I wanted to call them slabs but
> Ayal and Edu feel strongly about calling them volumes and the
> current naming system in general.

image and volume are overused everywhere and it would be extremely confusing to have multiple meanings to the same terms in the same system (we have image today which means virtual disk and volume which means a part of a virtual disk).
Personally I don't like the distinction between image and volume done in ec2/openstack/etc seeing as they're treated as different types of entities there while the only real difference is mutability (images are read-only, volumes are read-write).
To move to the industry terminology we would need to first change all references we have today to image and volume in the system (I would say also in ovirt-engine side) to align with the new meaning.
Despite my personal dislike of the terms, I definitely see the value in converging on the same terminology as the rest of the industry but to do so would be an arduous task which is out of scope of this discussion imo (patches welcome though ;)


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