VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
5. Virtual SAN software components
This section aims to provide a brief overview of some of the Virtual SAN software
components. Many of these components are not necessary to understand for day-today
use of Virtual SAN. However when it comes to troubleshooting, you may see
them referenced from time to time in logs, RVC commands and especially VSAN
Observer outputs. Therefore, for completeness sake, a brief overview is provided.
Local Log Structured Object Management - LSOM
LSOM works at the physical disk level, both flash devices and magnetic disks. It
handles the physical storage for Virtual SAN components on the local disks of the
ESXi hosts. It also handles the read caching and write buffering for the components.
Distributed Object Manager - DOM
DOM is responsible for the creation of virtual machine storage objects from local
components across multiple ESXi hosts in the Virtual SAN cluster by implementing
distributed RAID. It is also responsible for providing distributed data access paths to
these objects. There are 3 roles within DOM; client, owner and component manager.
Client: Provides access to an object. There may be multiple clients per object
depending on access mode.
Owner: Coordinates access to the object, including locking and object
configuration and reconfiguration. There is a single DOM owner per object.
All objects changes and writes go through the owner. Typically the client and
owner will reside on the same host, but this is not guaranteed and they may
reside on different hosts.
Component Manager: Interface for LSOM and the physical disks.
A node’s DOM may play any of the three roles for a single object
Cluster Level Object Manager - CLOM
CLOM ensures that an object has a configuration that matches its policy, i.e. stripe
width or failures to tolerate, to meet the requirements of the virtual machine. Each
ESXi host in a Virtual SAN cluster runs an instance of clomd, which is responsible for
the policy compliance of the objects. CLOM can be thought of as being responsible
for the placement of objects and their components.
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