[ovirt-devel] short recap of last vdsm call (15.4.2014)

[Adam Litke]

On 01/05/14 17:53 +0100, Dan Kenigsberg wrote:
>On Wed, Apr 30, 2014 at 01:26:18PM -0400, Adam Litke wrote:
>> On 30/04/14 14:22 +0100, Dan Kenigsberg wrote:
>> >On Tue, Apr 22, 2014 at 02:54:29PM +0300, ybronhei wrote:
>> >>hey,
>> >>
>> >>somehow we missed the summary of this call, and few "big" issues
>> >>were raised there. so i would like to share it with all and hear
>> >>more comments
>> >>
>> >>- task id in http header - allows engine to initiate calls with id
>> >>instead of following vdsm response - federico already started this
>> >>work, and this is mandatory for live merge feature afaiu.
>> >
>> >Adam, Federico, may I revisit this question from another angle?
>> >
>> >Why does Vdsm needs to know live-merge's task id?
>> >As far as I understand, (vmid, disk id) are enough to identify a live
>> >merge process.
>>
>> A vmId + diskId can uniquely identify a block job at a single moment
>> in time since qemu guarantees that only a single block job can run at
>> any given point in time.  But this gives us no way to differentiate
>> two sequential jobs that run on the same disk.  Therefore, without
>> having an engine-supplied jobID, we can never be sure if a one job
>> finished and another started since the last time we polled stats.
>
>Why would Engine ever want to initiate a new live merge of a
>(vmId,diskId) before it has a conclusive result of the previous
>success/failure of the previous attempt? As far as I understand, this
>should never happen, and it's actually good for the API to force
>avoidence of such a case.
>
>> Additionally, engine-supplied UUIDs is part of a developing framework
>> for next-generation async tasks.  Engine prefers to use a single
>> identifier to represent any kind of task (rather than some problem
>> domain specific combination of UUIDs).  Adhering to this rule will
>> help us to converge on a single implementation of ng async tasks
>> moving forward.
>
>I do not think that having a (virtual) table of
>    task_id -> vmId,diskId
>in Vdsm is much simpler than having it on the Engine machine.

It needs to go somewhere.  As the designers of the API we felt it
would be better for vdsm to hide the semantics of when a vmId,diskId
tuple can be considered a unique identifier.  If we ever do generalize
the concept of a transient task to other users (setupNetworks, etc) it
would be a far more consumable API if engine didn't need to handle a
bunch of special cases about what constitutes a "job ID" and the
specifics of its lifetime.  UUIDs are simple and already
well-supported.  Why make it more difficult than it has to be?

>
>I still find the nothion of a new framework for async tasks quite
>useful. But as I requested before, I think we should design it first,
>so it fits all conceivable users. In particular, if we should not tie it
>to the existence of a running VM. We'd better settle on persistence
>semantics that works for everybody (such as network tasks).
>
>Last time, the idea was struck down by Saggi and others from infra, who
>are afraid to repeat mistakes from the current task framework.

Several famous quotes apply here.  The only thing we have to fear is
fear itself :)  Sometimes perfect is the enemy of good.  Tasks
redesign was always going to be driven by the need to implement one
feature at first.  It just so happens that we volunteered to take a
stab at it for live merge.  It's clear that we won't be able to
completely replace the old tasks and get this feature out in one pass.
We believe the general principles of our tasks are generally
extensible to cover new use cases in the future:

 * Jobs are given an engine-supplied UUID when started
 * There is a well-known way to check if a job is running or not
 * There is a well-known way to test if a finished job succeeded or
   failed.

I believe we did spend quite a bit of time in March coming up with a
design for NG tasks.

Unfortunately it was infra who made our jobs vm-specific by requiring
the job status to be passed by getVMStats rather than an
object-agnostic getJobsStatus stand-alone API that could conglomerate
all job types into a single response.

>>
>> >
>> >If we do not have a task id, we do not need to worry on how to pass it,
>> >and where to persist it.
>>
>> There are at least 3 reasons to persist a block job ID:
>> * To associate a specific block job operation with a specific
>>  engine-initiated flow.
>> * So that you can clean up after a job that completed when vdsm could
>>  not receive the completion event.
>
>But if Vdsm dies before it managed to clean up, Engine would have to
>perform the cleanup via another host. So having this short-loop cleanup
>is redundant.
>

Fair enough.  We'll be doing the volume chain scan for every native VM
disk at VM startup.  The only exception is if we are recovering and
the running VM's recovery file does not show any outstanding block
jobs.  In that case we have definitive information that a rescan will
not be required.

>> * Since we must ask libvirt about block job events on a per VM, per
>>  disk basis, tracking the devices on which we expect block jobs
>>  enables us to eliminate wasteful calls to libvirt.
>
>This can be done by an in-memory cache.

Sure, but then you miss out on the other benefits I've enumerated.

>
>>
>> Hope this makes the rationale a bit clearer...
>
>Yes, but I am not yet convinced...
>

-- 
Adam Litke