VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
MTU & jumbo frames
Virtual SAN fully supports the use of jumbo frames, though it is not thought to
improve Virtual SAN network performance to any great extent. However recent
performance tests using Virtual SAN for VDI suggests that the use of jumbo frames
does assist with reduced CPU utilization. If jumbo frames is required, ensure that
the VMkernel interfaces and the virtual switches to which they connect are
configured to handle this new size. The physical switches to which the physical NICs
are connected also need to be configured appropriately.
A good test to verify that the hosts are able to communicate to each other over this
network with jumbo frames is to use the vmkping command but with the –s 9000
option (for the larger packet size) and have each of the hosts in the cluster ping each
other. Needless to say that if doesn’t work, no attempt should be made to enable
Virtual SAN until the network is configured correctly for jumbo frames.
Multicast traffic requirement
As stated in the introduction, multicast traffic is used for certain Virtual SAN cluster
functions. It is imperative that the Virtual SAN network facilitates multicast traffic
across all hosts participating in the Virtual SAN cluster. In version 5.5, this must be
over a layer-2 network. In 6.0, VMware supports multicast traffic routed over a
layer-3 network.
Virtual SAN uses a clustered metadata database and monitoring service (CMMDS) to
make particular metadata available to each host in the cluster. The CMMDS is
designed to be a highly available, performant and network efficient service that
shares information regarding host, network, disks, objects, components, etc. among
all of the hosts within the Virtual SAN cluster.
This distribution of metadata amongst all of the ESXi hosts in the cluster, and
keeping each host synchronized, could potentially consume a considerable amount
of compute resources and network bandwidth. Each ESXi host is intended to contain
an identical copy of this metadata which means that if general unicast forwarding
was used for this traffic, there would be duplicate traffic sent to all of the ESXi hosts
in the cluster.
Virtual SAN leverages multicast forwarding for the discovery of hosts and to
optimize network bandwidth consumption for the metadata updates from the
CMMDS service. This eliminates the computing resource and network bandwidth
penalties that unicast would impose in order to send identical data to multiple
recipients.
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