VMware : vSAN architecture components

The main components of vSAN architecture are the following:

  • Cluster-Level Object Manager (CLOM)
  • Distributed Object Manager (DOM)
  • Log Structured Object Manager (LSOM)
  • Cluster Membership, Monitoring, and Directory Services (CMMDS)
  • Reliable Datagram Transport (RDT)

The vSAN cluster can be compared to a house building where each component has a specific role.

The CLOM process runs on every vSAN node:

              • validates that objects can be created based on policies and available resources.
              • responsible for object compliance.
              • defines the creation and migration of objects.
              • distributes loads evenly between the vSAN nodes.
              • responsible for proactive and reactive rebalancing.
              • You manage the CLOM process with the /etc/init.d/clomd stop/start/restart command.



The DOM process runs on each host part of a vSAN cluster and receives instructions from the CLOM and the other DOMs running on other hosts:

              • all DOMs in a vSAN cluster resynchronize objects during a recovery.
              • each object has a DOM owner and a DOM client:
              • the DOM owner determines which processes are allowed to send I/O to the object.
              • the DOM client performs the I/O to an object on behalf of a particular virtual machine
              • the DOM client runs on every node that contains components
              • instructs the LSOM to create local components of an object:



The LSOM performs the following functions:

              • creates the local components as instructed by the DOM.
              • provides read and write buffering.
              • performs the encryption process for the vSAN datastore.
              • reports unhealthy storage and network devices.
              • performs I/O retries on failing devices.
              • interacts directly with the solid-state and magnetic devices.
              • performs solid-state drive log recovery when the vSAN node boots up


 The CMMDS performs the following functions:

              • provides topology and object configuration information to the CLOM and the DOM.
              • selects the owners of the objects.
              • inventories all cluster items, such as hosts, networks, and devices.
              • stores object metadata information, such as policy-related information on an in-memory database.
              • It defines the cluster roles: Master, Backup, and Agent.



Communication between all hosts in a vSAN cluster occurs every second. Updates are exchanged through the RDT, which is the network protocol for the transmission of vSAN traffic.

Hosts that are part of a vSAN cluster have one of the following roles, which are selected by vSAN during cluster discovery:

  • Master role: The host with the Master role receives all CMMDS updates from all hosts in the cluster.
  • Backup role: The host with the Backup role assumes the Master role if the current Master fails.
  • Agent role: Hosts with the Agent role can assume the backup role or the Master role as circumstances change. In clusters of four or more hosts, more than one host has the Agent role.

if you need to know which role is running on a specific host, run the following command on any host of the vSAN cluster:

$:~esxcli vsan cluster get

The next blog will be about the process of creating a virtual machine and how the interaction of all vSAN components.

Stay safe, stay connected.


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