[ovirt-users] What recovers a VM from pause?

Nicolas Ecarnot nicolas at ecarnot.net
Mon May 30 13:07:37 UTC 2016


We're planning a move from our old building towards a new one a few 
meters away.

In a similar way of Martijn 
(https://www.mail-archive.com/users@ovirt.org/msg33182.html), I have 
maintenance planed on our storage side.

Say an oVirt DC is using a SAN's LUN via iSCSI (Equallogic).
This SAN allows me to setup block replication between two SANs, seen by 
oVirt as one (Dell is naming it SyncRep).
Then switch all the iSCSI accesses to the replicated LUN.

When doing this, the iSCSI stack of each oVirt host notices the 
de-connection, tries to reconnect, and succeeds.
Amongst our hosts, this happens between 4 and 15 seconds.

When this happens fast enough, oVirt engine and the VMs don't even 
notice, and they keep running happily.

When this takes more than 4 seconds, there are 2 cases :

1 - The hosts and/or oVirt and/or the SPM (I actually don't know) 
notices that there is a storage failure, and pauses the VMs.
When the iSCSI stack reconnects, the VMs are automatically recovered 
from pause, and this all takes less than 30 seconds. That is very 
acceptable for us, as this action is extremely rare.

2 - Same storage failure, VMs paused, and some VMs stay in pause mode 
Manual "run" action is mandatory.
When done, everything recovers correctly.
This is also quite acceptable, but here come my questions :

My questions : (!)
- *WHAT* process or piece of code or what oVirt parts is responsible for 
deciding when to UN-pause a VM, and at what conditions?
That would help me to understand why some cases are working even more 
smoothly than others.
- Are there related timeouts I could play with in engine-config options?
- [a bit off-topic] Is it safe to increase some iSCSI timeouts of 
buffer-sizes in the hope this kind of disconnection would get un-noticed?


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