VMware vSAN – Understanding Fault Domains

VMware vSAN is one of the leading enterprise class software defined storage from VMware. It helps in leveraging the server based storage for enterprise applications. Advantages, as you might have already known – cost reduction, ease of administration and more…

In this post we are discussing one of the characteristic of vSAN, Fault Domains. What ?

Fault Domains helps an administrator to design the failure scenarios that may occur in a vSAN cluster. If a customer want to avoid data inaccessibility during a chassis failure or power failure in a rack etc… customer can do so by setting the right fault domains.

There should be a minimum of 3 fault domains for having this enabled on a cluster.

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How ?

In a vSAN cluster, writes will be send to multiple hosts/drives depending on the Storage policy and the Failures To Tolerate (FTT) settings. If the FTT=1, the write will be send to 2 hosts at the same time. Even if one of the host fails, the data will be still accessible as the replica will be available on the host and thus IO operation continues. We will discuss the IO operation in vSAN, in a separate post.

In case of Failure Domain configuration, the replicas will be saved in different Failure Domains. We can define all the hosts in the same rack to be part of one Failure Domain and thus data and its replica will never be in the same (host in the same) rack. Thus the administrator can plan for any maintenance activities at the rack level without any disruption of the services running on the vSAN.

Same applies for the chassis level or any other level protection. We can define all the fault domains at the chassis level, so that replicas will not reside in the same chassis.

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