VxRail – VSAN Software Component

I am starting new blog series on VxRail (Hyper Converged Product). To start with this, i thought about writing VxRail  – VSAN Software component which will help in better understanding of VxRail and VSAN.

VSAN Software Component

Cluster Level Object Manager (CLOM)

•An instance of CLOM runs on all the vSAN nodes
•/etc/init.d/clomd [start | stop | restart | status]  (Need to check on VxRail Appliance)
•Checks if object can be created based on available resources
•Ensures policy compliance of an object
•Is responsible for initial placement, migration and replacement of components
•Distributes the load equally across all the hosts

Distributed Object Manager (DOM)

•An instance of DOM runs on all the vSAN nodes
•Creates local components of an object once CLOM agrees to create an object
•Instructs DOM on other nodes to create the remaining components
•Resyncs the objects during recovery
•Defines three roles: Owner, Client and Component Manager

DOM Owner 

•Provides distributed access path to the Object
•There is one owner assigned to each DOM Object
•Owner also determines who is allowed to send I/Os to the object

DOM Client

•DOM Client performs I/Osto the object on behalf of a VM
•It runs on every host that runs a VM
•Typically resides with DOM Owner

Local Log-Structured Object Manager (LSOM)

•Performs SSD log recovery when the host boots
•Interacts with the MD and SSD directly
•Provides persistence of storage for the components
•Provides read and write buffering
•Reports unhealthy drives
•Performs I/O retries to the failing drives

•Reliable Datagram Transport (RDT)

•Network protocol for the transmission of vSAN traffic

•Cluster Membership, Monitoring and Directory Services (CMMDS)

•DOM, CLOM may use CMMDS database to learn about the topology and object configuration
•Requires multicast network to be able to form a cluster (pre vSAN 6.6)
•CMMDS is used to elect owners for objects
•It maintains inventory of items such as hosts, devices and networks
•Stores object metadata info such as policies, distributed RAID etc. in             an in-memory database
•CMMDS discovers, maintains and establishes a cluster of networked node members
•Defines four cluster roles: Master, Backup, Agent and witness host(forstretched clusters)
•The roles have nothing to do with managing objects in the cluster or doingI/Os to the objects
•The backup speeds up the process of convergence if master fails
•The roles are decided at the time of cluster discovery
•Updates are exchanged with the help of RDT
•Master to other hosts – >  (Every second)
•Other hosts to Master -> (Updates)

For more information about VSAN, please refer VMware VSAN