[ovirt-users] Data domain on engine host

[Alexander Wels]

On Friday, August 18, 2017 1:20:23 PM EDT Mitchell Smith wrote:
> Hi List,
> 
> I am in the process of deploying ovirt in a lab environment before
> implementing it in production.
> 
> My lab consists of three compute hosts and one engine host.
> 
> I am using the 4.2 nightly builds in this lab environment, mainly because I
> want to test the OVN functionality.
> 
> I wanted to add a NFS data domain on the engine host to store VM templates
> and ISO images, however when I go in to Storage -> Domains -> New and try
> and add a NFS domain, it only gives me the three compute hosts to create
> storage on, I’m guessing because the engine host doesn’t have VDSM
> installed.
> 
> Is there a way to do this, so I can make use of the considerable free space
> on the engine host, or do I have to add it as a compute host running VDSM,
> and if so how can I flag this host so no VMs get deployed on it, I only
> want to use the storage capacity.
> 
> Any feedback or suggestions would be very much appreciated.
> 
> Thanks in advance.
> 
> Mitch
> 

I think you are misunderstanding the architecture of oVirt a little bit so I 
will try to explain, and then I will address your particular issue. Basically 
the following happens:

1. The engine talks to the hosts (which are running VDSM). The engine itself 
doesn't actually know about or care about the storage at all, it just talks to 
the hosts. If storage operations need to happen it tells a special host, the 
SPM to do them. Only one host at a time is SPM.
2. The hosts talk to the storage through whatever mechanism is provided by the 
storage (FC, iSCSI, NFS, Gluster, etc). Management of the storage happens 
through the SPM host (this is to prevent simultanious operations that could 
corrupt the storage). This also means that under normal circumstances the 
available disk space on the hosts is not used (unless you are running some 
kind of hyper converged setup, which is a whole other story). Of course a lot 
of this is a little more nuanced than what I am saying, but in general the 
storage is considered separate from the hosts.

When you add a host to the engine you are simply saying this machine is 
available to RUN VMs and it should be able to communicate with the storage 
domains. Which leads to your question about why are you not seeing the storage 
available on the engine machine. In the NFS dialog you have to specify a HOST 
to use to connect to your storage (Not sure why, since I figured the SPM would 
be a good default, but you should ask the storage guys). Anyway I guess the 
host you pick will be the one that does the checks and mounts the NFS share 
and creates the storage domain on the NFS.

Now in your case you should create the NFS share on the engine machine, make 
sure it is visible to all your hosts, and pick one host from the list to 
connect to that share and configure the domain on it (You will have to specify 
the mount/etc). So at that point the machine with the engine, will be the 
engine machine AND the storage.

Now this is not a really good idea for production as when that machine goes 
down, you will have lost both your engine and your storage at the same time. 
Causing your VMs to pause, and you lost your ability to manage the environment 
because the engine is down.

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