On 12/05/2012 12:20 AM, Saggi Mizrahi wrote:
It can be much easier to understand the flow of a task in vdsm and
outside vdsm by a small schema, mainly for the each task's states.
To define the flow of a task you can separate between type of tasks
(network, storage, vms, or else), we should have task's states that
clarify if the task can be recovered or not, can be canceled or not and
As the only subsystem to use asynchronous tasks until now is the storage
subsystem I suggest going over how
I suggest tackling task creation, task stop, task remove and task recovery.
Other subsystem can create similar mechanisms depending on their needs.
There is no way of avoiding it, different types of tasks need different ways of
tracking\recovering from them.
network should always auto-recover because it can't get a "please fix" command
if the network is down.
Storage on the other hand should never start operations on it's own because it
might take up valuable resources from the host.
Tasks that need to be tracked on a single host, 2 hosts, or the entire cluster
need to have their own APIs.
VM configuration never persist across reboots, networking sometimes persists
and storage always persists.
This means that recovery procedures (from the managers point of view) need to
be vastly different.
Add policy, resource allocation, and error flows you see that VDSM doesn't have
nearly as much information to deal with the tasks.
----- Original Message -----
From: "Adam Litke" <a...@us.ibm.com>
To: "Saggi Mizrahi" <smizr...@redhat.com>
Cc: "VDSM Project Development" <firstname.lastname@example.org>, "engine-devel"
Baron" <aba...@redhat.com>, "Barak Azulay" <bazu...@redhat.com>, "Shireesh Anjal"
Sent: Tuesday, December 4, 2012 3:50:28 PM
Subject: Re: VDSM tasks, the future
On Tue, Dec 04, 2012 at 10:35:01AM -0500, Saggi Mizrahi wrote:
Because I started hinting about how VDSM tasks are going to look
I thought it's better I'll just write everything in an email so we
about it in context. This is not set in stone and I'm still
myself but it's very close to being done.
Don't debate them yourself, debate them here! Even better, propose
your idea in
schema form to show how a command might work exactly.
I don't like throwing ideas in the air
Canceling\Aborting\Reverting states should be more clarified and not
every state can lead to all types of states.
I tries to figure how task flow works today in vdsm, and this is what
I've got - http://wiki.ovirt.org/Vdsm_tasks
It looks like traditional timeout.. why not to split blocking actions
and non-blocking actions, non-blocking action will supply callback
function to return to if the task fails or success. for example:
- Everything is asynchronous. The nature of message based
that you can't have synchronous operations. This is not really
because it's just how TCP\AMQP\<messaging> works.
Can you show how a traditionally synchronous command might work?
Host.getVmList as an example.
The same as it works today, it's all a matter of how you wrap the transport
You will send a json-rpc request and wait for a response with the same id.
As for the bindings, there are a lot of way we can tackle that.
Always wait for the response and simulate synchronous behavior.
Make every method return an object to track the task.
task = host.getVmList()
if not task.wait(1):
res = task.result()
createAsyncTask(host.getVmList, params, timeout=30, callbackGetVmList)
Instead of using the dispatcher? Do you want to keep the dispatcher concept?
This is the reason why we can use the implementation of task as
something generic for all processes that we have.
of course things need different ways of tracking their progress...
That's why we need to use task's states with meaning, and split
storageTaskStates and networkTaskStates that inherit of TaskStates and
add their parts as in the new bootstrap implementation.
Also we can add hooks for each state as alonbl did in his otopi code
(not sure if we need that)
Have it both ways (it's auto generated anyway) and have
list = host.getVmList()
task = host.getVmList_async()
Have a high level and low level interfaces.
host = host()
req = host.sendRequest("123213", "getVmList", )
if not req.wait(1):
shost = SynchHost(host)
shost.getVmList() # Actually wraps a request object
ahost = AsyncHost(host)
task = getVmList() # Actually wraps a request object
- Task IDs will be decided by the caller. This is how json-rpc
works and also
makes sense because no the engine can track the task without
needing to have a
stage where we give it the task ID back. IDs are reusable as long
as no one
else is using them at the time so they can be used for
operations between clients (making sure a command is only executed
once on a
specific host without locking).
- Tasks are transient If VDSM restarts it forgets all the task
There are 2 ways to have persistent tasks: 1. The task creates an
you can continue work on in VDSM. The new storage does that by the
copyImage() returns one the target volume has been created but
before the data
has been fully copied. From that moment on the stat of the copy
queried from any host using getImageStatus() and the specific copy
can be queried with getTaskStatus() on the host performing it.
crashes, depending on policy, either VDSM will create a new task to
the copy or someone else will send a command to continue the
that will be a new task. 2. VDSM tasks just start other operations
not through the task interface. For example Gluster.
gluster.startVolumeRebalance() will return once it has been
Gluster. glster.getOperationStatuses() will return the state of
from any host. Each call is a task in itself.
I worry about this approach because every command has a different
checking progress. For migration, we have to check VM status on the
dest hosts. For image copy we need to use a special status call on
image. It would be nice if there was a unified method for checking
operation. Maybe that can be completion events.
<-- Operation Started
Wait for event ...
<-- Event: Operation <id> done <code>
For an early error:
<-- Error: <code>
The thing is that a lot of things need a different way of tracking their
Storage have completely different semantics from network or VM operations.
Like for instance: general states can be - starting, started, finishing,
finished, and each specific implementation adds middle states. like
waitForResource, processing, recovering and inc..
for each one you can add levels (pre state, post state) that can add
That way Task Object will be a general way to implement specific
process, you will have a NetworkTask and StorageTask and the
infrastructure will be the interface and implementation of the generic
So here how vdsm can work that way:
image.copy() ---> copyImage::starting (same starting code - keeping the
id, and move forward to next state)
copyImage::started (waiting to recovery
file that task is started)
copyImage::part1 (whatever you want to do)
copyImage::part2 (whatever you want to do)
copyImage::part3 (whatever you want to
do) -- for each process the programmers will add their states as they
want in a sequence flow
result <------ copyImage::finishing (send back to client a
success and clean recovery file)
copyImage::finished (sign task id as
If somewhere in the middle an error occurred, it easier to start over
and remember where we were.
The problem with that is that we need to modify the current
implementation for each process, and I'm not sure if we want to get
there.. but if we do, it won't be so hard.
We can split the logic of each process to define a logic of each state,
and then arranging the states flow for each process and clarify what can
be recovered or not, what signs corruption or errors, and how the
returned result can point of the current process status (\state)
- No task tags. They are silly and the caller can mangle whatever
in the task
ID if he really wants to tag tasks.
- No explicit recovery stage. VDSM will be crash-only, there
efforts to make everything crash-safe. If that is problematic, in
networking, VDSM will recover on start without having a task for
How does this work in practice for something like creating a new
image from a
- No clean Task: Tasks can be started by any number of hosts this
there is no way to own all tasks. There could be cases where VDSM
tasks on it's own and thus they have no owner at all. The caller
continually track the state of VDSM. We will have brodcasted events
If a disconnected client might have missed a completion event, it
will need to
check state. This means each async operation that changes state must
proceedure for checking progress of a potentially ongoing operation.
Image.copy, that process would be to lookup the new image and check
- No revert Impossible to implement safely.
How do the engine folks feel about this? I am ok with it :)
I don't care, unless they find a way to change they way logic works they can't
The whole concept of recovery (as it is defined now) doesn't work in an HA
- No SPM\HSM tasks SPM\SDM is no longer necessary for all domain
for type). What used to be SPM tasks, or tasks that persist and
restarted on other hosts is talked about in previous bullet points.
A nice simplification.
Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center
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