[ovirt-users] 4-node oVirt with replica-3 gluster

[Sahina Bose]

On Tue, Sep 27, 2016 at 8:59 PM, Goorkate, B.J. <b.j.goorkate at umcutrecht.nl>
wrote:

> Hi Sahina,
>
> First: sorry for my delayed response. I wasn't able to respond earlier.
>
> I already planned on adding the 4th node as a gluster client, so thank you
> for
> confirming that this works.
>
> Why I was in doubt is that certain VMs with a lot of storage I/O on the
> 4th node have to
> replicate to 3 other hosts (the replica-3 gluster nodes) over the storage
> network, while
> a VM on 1 of the replica-3 gluster nodes only has to replicate to two
> other nodes over
> the network, thus creating less network traffic.
>
> Does this make sense?
>
> And if it does: can that be an issue?
>

IIUC, the 4th node that you add to the cluster is serving only compute and
there is no storage (bricks) capacity added. In this case, yes, all reads
and writes are over the network - this is like a standard oVirt deployment
where storage is over the network (non hyper converged).
While thoeretically this looks like an issue, it may not be, as there are
multiple factors affecting performance. You will need to measure the impact
on guest performance when VMs run on this node and see if it is acceptable
to you. One thing you could do is schedule VMs  that do not have stringent
perf requirements on the 4th node?

There are also improvements planned in upcoming releases of gluster which
improve the I/O performance further (compound FOPS, libgfapi access), so
whatever you see now should improve further.


> Regards,
>
> Bertjan
>
> On Fri, Sep 23, 2016 at 04:47:25PM +0530, Sahina Bose wrote:
> >
> >
> > On Fri, Sep 23, 2016 at 4:14 PM, Davide Ferrari <davide at billymob.com>
> wrote:
> >
> >     I'm struggling with the same problem (I say struggling because I'm
> still
> >     having stability issues for what i consider a stable cluster) but
> you can:
> >     - create a replica 3 engine gluster volume
> >     - create replica 2 data, iso and export volumes
> >
> >
> > What are the stability issues you're facing? Data volume if used as a
> data
> > storage domain should be a replica 3 volume as well.
> >
> >
> >
> >     Deploy the hosted-engine on the first VM (with the engine volume)
> froom the
> >     CLI, then log in Ovirt admin, enable gluster support, install *and
> deploy*
> >     from the GUI host2 and host3 (where the engine bricks are) and then
> install
> >     host4 without deploying. This should get you the 4 hosts online, but
> the
> >     engine will run only on the first 3
> >
> >
> > Right. You can add the 4th node to the cluster, but not have any bricks
> on this
> > volume in which case VMs will be run on this node but will access data
> from the
> > other 3 nodes.
> >
> >
> >
> >     2016-09-23 11:14 GMT+02:00 Goorkate, B.J. <
> b.j.goorkate at umcutrecht.nl>:
> >
> >         Dear all,
> >
> >         I've tried to find a way to add a 4th oVirt-node to my existing
> >         3-node setup with replica-3 gluster storage, but found no usable
> >         solution yet.
> >
> >         From what I read, it's not wise to create a replica-4 gluster
> >         storage, because of bandwith overhead.
> >
> >         Is there a safe way to do this and still have 4 equal oVirt
> nodes?
> >
> >         Thanks in advance!
> >
> >         Regards,
> >
> >         Bertjan
> >
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> >     --
> >     Davide Ferrari
> >     Senior Systems Engineer
> >
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