[Engine-devel] [vdsm] VDSM tasks, the future

[ybronhei]

On 12/05/2012 12:20 AM, Saggi Mizrahi wrote:
> 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" <agl at us.ibm.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>, "Ayal
>> Baron" <abaron at redhat.com>, "Barak Azulay" <bazulay at redhat.com>, "Shireesh Anjal" <sanjal at redhat.com>
>> 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
>>> going forward
>>> I thought it's better I'll just write everything in an email so we
>>> can talk
>>> about it in context.  This is not set in stone and I'm still
>>> debating things
>>> 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
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 
inc..

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
>>> - Everything is asynchronous.  The nature of message based
>>> communication is
>>> that you can't have synchronous operations.  This is not really
>>> debatable
>>> because it's just how TCP\AMQP\<messaging> works.
>> Can you show how a traditionally synchronous command might work?
>>   Let's take
>> Host.getVmList as an example.
> The same as it works today, it's all a matter of how you wrap the transport layer.
> 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):
>      task.cancel()
> else:
>      res = task.result()
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:

createAsyncTask(host.getVmList, params, timeout=30, callbackGetVmList)

Instead of using the dispatcher? Do you want to keep the dispatcher concept?

> 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()
> host.connect("tcp://host:3233")
> 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
>>> synchronizing
>>> 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
>>> information.
>>> There are 2 ways to have persistent tasks: 1. The task creates an
>>> object that
>>> you can continue work on in VDSM.  The new storage does that by the
>>> fact that
>>> 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
>>> can be
>>> queried from any host using getImageStatus() and the specific copy
>>> operation
>>> can be queried with getTaskStatus() on the host performing it.
>>>   After VDSM
>>> crashes, depending on policy, either VDSM will create a new task to
>>> continue
>>> the copy or someone else will send a command to continue the
>>> operation and
>>> that will be a new task.  2. VDSM tasks just start other operations
>>> track-able
>>> not through the task interface. For example Gluster.
>>> gluster.startVolumeRebalance() will return once it has been
>>> registered with
>>> Gluster.  glster.getOperationStatuses() will return the state of
>>> the operation
>>> from any host.  Each call is a task in itself.
>> I worry about this approach because every command has a different
>> semantic for
>> checking progress.  For migration, we have to check VM status on the
>> src and
>> dest hosts.  For image copy we need to use a special status call on
>> the dest
>> image.  It would be nice if there was a unified method for checking
>> on an
>> operation.  Maybe that can be completion events.
>>
>> Client:               vdsm:
>> -------               -----
>>
>> Image.copy(...)  -->
>>                   <--  Operation Started
>> Wait for event   ...
>>                   <--  Event: Operation <id> done <code>
>>
>> For an early error:
>>
>> Client:               vdsm:
>> -------               -----
>>
>> Image.copy(...)  -->
>>                   <--  Error: <code>
>>
>>
> The thing is that a lot of things need a different way of tracking their progress.
> Storage have completely different semantics from network or VM operations.
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)

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 
more flexibility.

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 
parts.

So here how vdsm can work that way:
client:            vsdm:
--------            ---------
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 
succeeded)

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.
>> Yes.  Agreed.
>>
>>> - No explicit recovery stage.  VDSM will be crash-only, there
>>> should be
>>> efforts to make everything crash-safe.  If that is problematic, in
>>> case of
>>> networking, VDSM will recover on start without having a task for
>>> it.
>> How does this work in practice for something like creating a new
>> image from a
>> template?
>>
>>> - No clean Task: Tasks can be started by any number of hosts this
>>> means that
>>> there is no way to own all tasks.  There could be cases where VDSM
>>> starts
>>> tasks on it's own and thus they have no owner at all.  The caller
>>> needs to
>>> continually track the state of VDSM. We will have brodcasted events
>>> to
>>> mitigate polling.
>> 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
>> document a
>> proceedure for checking progress of a potentially ongoing operation.
>>   For
>> Image.copy, that process would be to lookup the new image and check
>> its state.
>>
>>> - 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 have it.
> The whole concept of recovery (as it is defined now) doesn't work in an HA cluster.
>>> - No SPM\HSM tasks SPM\SDM is no longer necessary for all domain
>>> types (only
>>> for type).  What used to be SPM tasks, or tasks that persist and
>>> can be
>>> restarted on other hosts is talked about in previous bullet points.
>>>
>> A nice simplification.
>>
>>
>> --
>> Adam Litke <agl at us.ibm.com>
>> IBM Linux Technology Center
>>
>>
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-- 
Yaniv Bronhaim.
RedHat, Israel
09-7692289
054-7744187

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