Open Stack

On Sun, 2021-01-17 at 11:51 +0100, Nagaraj Akkina wrote:
> Hello,
> 
> Openstack host show compute-host, shows more memory allocated to instances
> than actual physical memory, although the memory allocation ratio is 1.0 in
> nova.conf, the same is also shown in horizon dashboard hypervisor statistics.
> We can see this in many compute nodes in the cluster. Also these statistics
> does not match with free -g or cat /proc/meminfo.
> 
> For Example :                                  Memory_mb
> | compute-node23 | (total)                     |  257915 |    
> | compute-node23 | (used_now)           |  271360 |    
> | compute-node23 | (used_max)           |  267264 |    
> 
> 
> How to avoid misleading statistics, also which statistics are actually
> correct?
> 
> versions :
>  openstack : Stein
>  nova : 2:19.3.0-0ubuntu1~cloud0
> 
> Regards,
> Akkina 

As you've noted, these statistics are generally misleading or outright
incorrect. They're so often wrong, in fact, that we have removed them from the
API starting with the 2.88 API microversion, which is being introduced in the
next release (Wallaby). There are two suggestions. Firstly, Placement provides
far better information as to the state of all the things nova knows about and
should be used instead. You can find examples of how to replace this using the
following link:

https://specs.openstack.org/openstack/nova-specs/specs/wallaby/approved/modernize-os-hypervisors-api.html

Secondly, the fact that you have overcommit enabled and are still seeing this
happen suggests you don't have enough memory reserved for non-VM processes and
other overhead. This is configured using the '[DEFAULT]
reserved_host_memory_mb', config option, which defaults to a mere 512MB. You may
need to increase this, basing your values on the average memory consumption of
the host with and without load.

For reference, the way that 'memory_mb_used' is calculated is by subtracting the
total amount free memory reported via '/proc/meminfo' from the total amount of
memory reported by libvirt.

Hope this helps,
Stephen
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