[ovirt-devel] [vdsm] VM recovery now depends on HSM

[Adam Litke]

On 09/07/14 13:11 +0200, Michal Skrivanek wrote:
>
>On Jul 8, 2014, at 22:36 , Adam Litke <alitke at redhat.com> wrote:
>
>> Hi all,
>>
>> As part of the new live merge feature, when vdsm starts and has to
>> recover existing VMs, it calls VM._syncVolumeChain to ensure that
>> vdsm's view of the volume chain matches libvirt's.  This involves two
>> kinds of operations: 1) sync VM object, 2) sync underlying storage
>> metadata via HSM.
>>
>> This means that HSM must be up (and the storage domain(s) that the VM
>> is using must be accessible.  When testing some rather eccentric error
>> flows, I am finding this to not always be the case.
>>
>> Is there a way to have VM recovery wait on HSM to come up?  How should
>> we respond if a required storage domain cannot be accessed?  Is there
>> a mechanism in vdsm to schedule an operation to be retried at a later
>> time?  Perhaps I could just schedule the sync and it could be retried
>> until the required resources are available.
>
>I've briefly discussed with Federico some time ago that IMHO the syncVolumeChain needs to be changed. It must not be part of VM's create flow as I expect this quite a bottleneck in big-scale environment (it is now in fact not executing only on recovery but on all 4 create flows!).
>I don't know how yet, but we need to find a different way. Now you just added yet another reason.
>
>So…I too ask for more insights:-)

Sure, so... We switched to running syncVolumeChain at all times to
cover a very rare scenario:

1. VM is running on host A
2. User initiates Live Merge on VM
3. Host A experiences a catastrophic hardware failure before engine
can determine if the merge succeeded or failed
4. VM is restarted on Host B

Since (in this case) the host cannot know if a live merge was in
progress on the previous host, it needs to always check.


Some ideas to mitigate:
1. When engine recreates a VM on a new host and a Live Merge was in
progress, engine could call a verb to ask the host to synchronize the
volume chain.  This way, it only happens when engine knows it's needed
and engine can be sure that the required resources (storage
connections and domains) are present.

2. The syncVolumeChain call runs in the recovery case to ensure that
we clean up after any missed block job events from libvirt while vdsm
was stopped/restarting.  In this case, the block job info is saved in
the vm conf so the recovery flow could be changed to query libvirt for
block job status on only those disks where we know about a previous
operation.  For those found gone, we'd call syncVolumeChain.  In this
scenario, we still have to deal with the race with HSM initialization
and storage connectivity issues.  Perhaps engine should drive this
case as well?


-- 
Adam Litke