9:35 a.m.
On Sat, 7 Jul 2018 16:28:49 +0200
Gianluca Cecchi <gianluca.cecchi(a)gmail.com> wrote:
Hello,
I'm testing a virtual rhcs cluster based on 4 nodes that are CentOS
7.4 VMs. So the stack is based on Corosync/Pacemaker
I have two oVirt hosts and so my plan is to put two VMs on first host
and two VMs on the second host, to simulate a two sites config and
site loss, before going to physical production config.
Incidentally the two hypervisor hosts are indeed placed into different
physical datacenters.
So far so good.
I decided to use OVN for the intracluster dedicated network
configured for corosync (each VM has two vnics, one on production lan
and one for intracluster).
I detected that the cluster worked and formed (also only two nodes)
only if the VMs run on the same host, while it seems they are not
able to communicate when on different hosts. Ping is ok and an
attempt of ssh session between them on intracluster lan, but cluster
doesn't come up So after digging in past mailing list mails I found
this recent one:
https://lists.ovirt.org/archives/list/users@ovirt.org/thread/RMS7XFOZ67O3...
where the solution was to set 1400 for the MTU of the interfaces on
OVN network.
It seems it resolves the problem also in my scenario:
- I live migrated two VMs on the second host and rhcs clusterware
didn't complain
- I relocated a resource group composed by several LV/FS, VIP and
application from VM running on host1 to VM running on host2 without
problems.
There will be a new feature [1][2] about propagating the MTU of the
logical network into the guest.
In ovirt-4.2.5 the logical network MTU <= 1500 will be propagated for
clusters with switch type OVS and linux bridge, and MTU > 1500 will be
propagated only for clusters with switch type linux bridge, if the
requirements [3] are fulfilled in oVirt >= 4.2.5. OVS clusters will
work for MTU > 1500 latest in oVirt 4.3.
In this new feature a new default config setting "MTU for tunneled
networks" is introduced, which will be set initially to 1442.
So the question is: can anyone confirm what are guidelines for
settings vnics on OVN?
In the context of oVirt, I am only aware of [1] and [4].
Starting from oVirt 4.1 you can activate the OVN's internal dhcp server
by creating a subnet for the network [4]. The default configuration will
offer a MTU of 1442 to the guest, which is optimal for GENEVE tunneled
networks over physical networks with a MTU of 1500.
Is there already a document in place about MTU
settings for OVN based vnics?
There are some documents about MTU in OpenStack referenced in [1].
Other particular settings or
limitations if I want to configure a vnic on OVN?
libvirt's network filters are not applied to OVN networks, so you
should disable network filtering in oVirt's vNIC profile. This is
tracked in [5].
[1]
https://ovirt.org/develop/release-management/features/network/managed_mtu...
[2]
https://github.com/oVirt/ovirt-site/pull/1667
[3]
https://ovirt.org/develop/release-management/features/network/managed_mtu...
[4]
https://github.com/oVirt/ovirt-provider-ovn/#section-dhcp
[5]
https://bugzilla.redhat.com/show_bug.cgi?id=1502754
Thanks,
Gianluca