2012-12-11 4:36, Saggi Mizrahi:


----- Original Message -----
From: "Adam Litke" <a...@us.ibm.com>
To: "Saggi Mizrahi" <smizr...@redhat.com>
Cc: "Shu Ming" <shum...@linux.vnet.ibm.com>, "engine-devel" <engine-de...@ovirt.org>, 
"VDSM Project Development"
<vdsm-devel@lists.fedorahosted.org>
Sent: Monday, December 10, 2012 1:39:51 PM
Subject: Re: [vdsm] RFC: New Storage API

On Thu, Dec 06, 2012 at 11:52:01AM -0500, Saggi Mizrahi wrote:

----- Original Message -----
From: "Shu Ming" <shum...@linux.vnet.ibm.com>
To: "Saggi Mizrahi" <smizr...@redhat.com>
Cc: "VDSM Project Development"
<vdsm-devel@lists.fedorahosted.org>, "engine-devel"
<engine-de...@ovirt.org>
Sent: Thursday, December 6, 2012 11:02:02 AM
Subject: Re: [vdsm] RFC: New Storage API

Saggi,

Thanks for sharing your thought and I get some comments below.


Saggi Mizrahi:
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.

Where does repoID come from? I think repoID doesn't exist before
connectStorageRepository() return.  Isn't repoID a return value
of
connectStorageRepository()?
No, repoIDs are no longer part of the domain, they are just a
transient handle.
The user can put whatever it wants there as long as it isn't
already taken by another currently connected domain.
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
I think we will also need a function to check if a a VirtualDisk
is
based on a specific snapshot.
Like: isSnapshotOf(virtualDiskId, baseSnapshotID):
No, the design is that volume dependencies are an implementation
detail.
There is no reason for you to know that an image is physically a
snapshot of another.
Logical snapshots, template information, and any other information
can be set by the user by using the userData field available for
every image.
Statements like this make me start to worry about your userData
concept.  It's a
sign of a bad API if the user needs to invent a custom metadata
scheme for
itself.  This reminds me of the abomination that is the 'custom'
property in the
vm definition today.
In one sentence: If VDSM doesn't care about it, VDSM doesn't manage it.

userData being a "void*" is quite common and I don't understand why you would 
thing it's a sign of a bad API.
Further more, giving the user choice about how to represent it's own metadata 
and what fields it want to keep seems reasonable to me.
Especially given the fact that VDSM never reads it.

The reason we are pulling away from the current system of VDSM understanding 
the extra data is that it makes that data tied to VDSMs on disk format.
VDSM on disk format has to be very stable because of clusters with multiple 
VDSM versions.
Further more, since this is actually manager data it has to be tied to the 
manager backward compatibility lifetime as well.
Having it be opaque to VDSM ties it to only one, simpler, support lifetime 
instead of two.

Making userData being opaque gives flexibilities to the management applications. To me, opaque userDaa can have two types at least. The first is the userData for runtime only. The second is the userData expected to be persisted into the metadata disk. For the first type, the management applications can store their own data structures like temporary task states, VDSM query caches &etc. After the VDSM host is fenced, the userData goes away. For the second type, different management applications can have different formats of userData and persisting it to the VDSM metadata disk will corrupt each other in very high possibility. You may argue that, all the management applications should reach a agreement before they use it as the second type. But in practice, it is pretty hard to maintain.



I guess you are implying that it will make it problematic for multiple users to 
read userData left by another user because the formats might not be compatible.
The solution is that all parties interested in using VDSM storage agree on 
format, and common fields, and supportability, and all the other things that 
choosing a supporting *something* entails.
This is, however, out of the scope of VDSM. When the time comes I think how the 
userData blob is actually parsed and what fields it keeps should be discussed 
on ovirt-devel or engine-devel.

The crux of the issue is that VDSM manages only what it cares about and the 
user can't modify directly.
This is done because everything we expose we commit to.
If you want any information persisted like:
- Human readable name (in whatever encoding)
- Is this a template or a snapshot
- What user owns this image

You can just put it in the userData.
VDSM is not going to impose what encoding you use.
It's not going to decide if you represent your users as IDs or names or ldap 
queries or Public Keys.
It's not going to decide if you have explicit templates or not.
It's not going to decide if you care what is the logical image chain.
It's not going to decide anything that is out of it's scope.
No format is future proof, no selection of fields will be good for any 
situation.
I'd much rather it be someone else's problem when any of them need to be 
changed.
They have currently been VDSMs problem and it has been hell to maintain.

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
Does this function mean that we can copy the image from one
repository
to another repository? Does it cover the semantics of storage
migration,
storage backup, storage incremental backup?
Yes, the main purpose is copying to another repo. and you can even
do incremental backups.
Also the following flow
1. Run a VM using imageA
2. write to disk
3. Stop VM
4. copy imageA to repoB
5. Run a VM using imageA again
6. Write to disk
7. Stop VM
8. Copy imageA again basing it of imageA_copy1 on repoB creating a
diff on repo diff without snapshotting the original image.

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.
What does the "represent" mean here?
Anything, but mostly image formate RAW\QCOW2 when performance
strategy has been selected.
"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
Any host can optimize the image? No need to be SDM?
On anything but lvm based block domains there will not even be an
SDM.
On SDM based domains we will try as hard as we can to have as many
operations executable on any host.
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.
So we need a function to know what tasks are running on the image
getImageStatus()
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.
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--
---
舒明 Shu Ming
Open Virtualization Engineerning; CSTL, IBM Corp.
Tel: 86-10-82451626  Tieline: 9051626 E-mail: shum...@cn.ibm.com
or
shum...@linux.vnet.ibm.com
Address: 3/F Ring Building, ZhongGuanCun Software Park, Haidian
District, Beijing 100193, PRC



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--
Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center




--
---
舒明 Shu Ming
Open Virtualization Engineerning; CSTL, IBM Corp.
Tel: 86-10-82451626  Tieline: 9051626 E-mail: shum...@cn.ibm.com or 
shum...@linux.vnet.ibm.com
Address: 3/F Ring Building, ZhongGuanCun Software Park, Haidian District, 
Beijing 100193, PRC


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