6:37 p.m.
For my home-lab I operate a 3 node HCI cluster on 100% passive Atoms, mostly to run light
infrastructure services such as LDAP and NextCloud.
I then add workstations or even laptops as pure compute hosts to the cluster for bigger
but temporary things, that might actually run a different OS most of the time or just be
shut off. From oVirt's point of view, these are just first put into maintenance and
then shut down until needed again. No fencing or power management, all manual.
All nodes, even the HCI ones, run CentOS7 with more of a workstation configuration, so
updates pile up pretty quickly.
After I recently upgraded one of these extra compute nodes, I found my three node HCI
cluster not just faltering, but indeed very hard to reactivate at all.
The faltering is a distinct issue: I have the impression that reboots of oVirt nodes cause
broadcast storms on my rather simplistic 10Gibt L2 switch, which a normal CentOS instance
(or any other OS) doesn't, but that's for another post.
No what struck me, was that the gluster daemons on the three HCI nodes kept complaining
about a lack of quorum long after the network was all back to normal, even if all three of
them were there, saw each other perfectly on "gluster show status all", ready
and without any healing issues pending at all.
Glusterd would complain on all three nodes that there was no quota for the bricks and stop
them.
That went away as soon as I started one additional compute node, a node that was a gluster
peer (because an oVirt host added to a HCI cluster always gets put into the Gluster, even
if it's not contributing storage) but had no bricks. Immediately the gluster daemon on
the three nodes with contributing bricks would report back good quota and launch the
volumes (and thus all the rest of oVirt), even if in terms of *storage bricks* nothing had
changed.
I am afraid that downing the extra compute-only oVirtNode will bring down the HCI: Clearly
not the type of redundancy it's designed to deliver.
Evidently such compute-only hosts (and gluster members) get included into some quorum
deliberations even if they hold not a single brick, neither storage nor arbitration.
To me that seems like a bug, if that is indeed what happens: There I need your advice and
suggestions.
AFAIK HCI is a late addition to oVirt/RHEV as storage and compute were orginally designed
to be completely distinct. In fact there are still remnants of documentation which seem to
prohibit using a node for both compute and storage... what HCI is all about.
And I have seen compute nodes with "matching" storage (parts of a distinct HCI
setup, that was taken down but still had all the storage and Gluster elements operable),
being happliy absorbed into a HCI cluster with all Gluster storage appearing in the GUI
etc., without any manual creation or inclusion of bricks: Fully automatic (and
undocumented)!
In that case it makes sense to widen the scope of quota calculations when additional nodes
are hyperconverged elements with contributing bricks. It also seems the only way to turn a
3 node HCI into 6 or 9 node one.
But if you really just want to add compute nodes without bricks, those can't get
"quota votes" without storage to play a role in the redundancy.
I can easily imagine the missing "if then else" in the code here, but I was
actually very surprised to see those failure and success messages coming from glusterd
itself, which to my understanding is pretty unrelated to oVirt on top. Not from the
management engine (wasn't running anyway), not from VDSM.
Re-creating the scenario is very scary even if I have gone through this three times
already, trying to just bring my HCI back up. And then there is so verbose logs all over
the place that I'd like some advice which ones I should post.
But simply speaking: Gluster peers should get no quota voting rights on volumes unless
they contribute bricks. That rule seems broken.
Those in the know, please let me know if am on a goose chase or if there is a real issue
here that deserves a bug report.