На 31 май 2020 г. 15:52:14 GMT+03:00, aigin...@gmail.com написа:
>Hi,
>
>Our company uses Ovirt to host some of its virtual machines. The
>version used is 4.2.6.4-1.el7. There are about 36 virtual hosts in it.
>The specifications used for the host machine is 30G RAM and 6 CPUs.
>Some of the VMs in the ovirt host run with 4 CPUs. Some with 2 CPUs. 
>
>The problem I face now is that recently there was a need for high CPU
>and memory specs to setup a VM for DR. I created a VM with 16G RAM and
>6 CPUs, without checking the CPUs available in the host first. After
>DR, the VM was brought down already. Then later another person in the
>team brought the VM back up for a different DR use, for a much larger
>DB restoration purpose.
>
>This caused the VM to pause due to storage error. And then worse things
>happened, whereby 2 other VMs inadvertently went down. Although I
>assumed that this was caused by storage errors/problems, the senior
>admins in the team concluded that the problem was due to fencing
>because of the max allotted CPU for the host being used for the VM.

 Check the libvirt logs  on the host where the VM was  running. In the engine, 
you can check the logs  for any fencing, but I have never seen such thing as 
"excessive CPU allocation" to cause fencing.
Either the VM passes the checks (overcommit rules,  scheduling,etc)  and gets 
up and running  or the engine will refuse  to power  it up.
Also  check via  journalctl  for any messages  at that time  for the 
'sanlock.service' .  Any issues  (storage  unavailable or high lattency 
detected)  will be reported  via the sanlock service  on the affected node.
If you use multipath  -  check if it also reported  any paths failing.


>Now what I need to know is how to properly allocate CPU resources to a
>host to run multiple virtual machines in it like the situation above. 
The  best way is to start with less  CPUs  as possible.
Here is a  short (or maybe not)  example:
Hoypervisor has  8 CPUs/8 Threads. First VM has 1  CPU. Second has 6 CPUs 
allocated and a third VM has  8 CPUs allocated.
For the hypervisor to allocate CPU time for the third  (beefy) VM,  it needs to 
have all 8 CPUs  available. As the host itself has 8 cores and usually some  OS 
stuff is going on - the third  VM  will receive  far less  CPU time than 
first/second VM.

>I even tried to look for errors in vdsm.log, but this log was not
>available in the host machine nor in the affected VM. My colleague
>asked me to check "Events" section of the ovirt management interface to
>see past the past events. However, I don't find much details about the
>fencing activity or how the fencing occurred or what caused the
>fencing. 
>
>And how did they conclude that the CPU count caused the fencing and not
>the storage?

Interesting question... I think they just assumed. In worst case  scenario (CPU 
 starvation), the vdsmd service  might not respond to the engine, but then a 
'soft' reboot will happen where the engine will restart  this service over ssh.
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Best  Regards,
Strahil  Nikolov
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