VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Introduction to Virtual SAN Networking
Before getting into network in detail, it is important to understand the roles that
nodes/hosts can play in Virtual SAN. There are three roles in Virtual SAN: master,
agent and backup. There is one master that is responsible for getting CMMDS
(clustering service) updates from all nodes, and distributing these updates to agents.
Roles are applied during cluster discovery, when all nodes participating in Virtual
SAN elect a master. A vSphere administrator has no control over roles.
A backup node will assume the master role should the master fail. This will avoid a
complete rediscovery of every node, object and component in the cluster, as the
backup role will already have a full copy of the directory contents, and can
seamlessly assume the role of master, speeding up recovery in the event of a master
failure.
There are a number of distinct parts to Virtual SAN networking. First there is the
communication that takes place between all of the ESXi hosts in the Virtual SAN
cluster, indicating that they are still actively participating in Virtual SAN. This is
done via multicast traffic, and a heartbeat is sent from the master to all hosts once
every second to ensure they are still active.
There is also communication between the master and the backup and agent nodes,
where the master keeps the other nodes up to date with regards to the configuration
of the cluster. This is also multicast traffic, but is relatively light from a traffic
volume perspective.
Lastly, there is virtual machine disk I/O. This makes up the majority of the traffic on
the Virtual SAN network. VMs on the Virtual SAN datastore are made up of a set of
objects. These objects may be made up of one or more components, for example a
number of RAID-1 stripes or a number of RAID-0 mirrors.
Invariably, a VMs compute and a VM’s storage will be located on different ESXi hosts
in the cluster. It may also transpire that if a VM has been configured to tolerate one
or more failures, the compute may be on node 1, the first RAID-0 mirror may be on
host 2 and the second RAID-0 mirror could be on host 3. In this case, disk reads and
writes for this virtual machine will have to traverse the Virtual SAN network. This is
unicast traffic, and forms the bulk of the Virtual SAN network traffic.
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