VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
component that is marked as DEGRADED are both considered a single failure. If a
VM is configured to tolerate a single failure, a VM remains available in the event of a
component going DEGRADED or ABSENT.
In order to rebuild components when a failure occurs, Virtual SAN will search for
hosts that satisfy placement rules like that 2 mirrors may not share hosts or fault
domains. It also looks for disks with free disk space.
If such hosts and disks are found, Virtual SAN will create new components on those
hosts and disks, and start the recovery process if there is enough capacity and
resources in the cluster. However, the exact timing of this behavior depends on the
failure state of the component. If the component is DEGRADED, i.e. Virtual SAN
believes the data on that component is lost and won't return, Virtual SAN takes
immediate action and starts the rebuild at once. If the component is ABSENT,
implying Virtual SAN believes that the data may come back, the following
optimization kicks in.
Virtual SAN has to tradeoff two aspects: On the one hand, Virtual SAN wants to
minimize the time that virtual machines are exposed to risk due to reduced
redundancy. On the other hand, a full rebuild of a component takes time and
resources. If the disruption is short (minutes), it’s better for Virtual SAN to just wait
and allow the situation to rectify itself (e.g. reboot of host). If the disruption
continues for long (hours), Virtual SAN should restore full redundancy even if the
component will eventually come back. Thus, Virtual SAN uses a configurable timer
of 60 minutes. If a component has been ABSENT for 60 minutes, Virtual SAN will
proceed to replace it with a new component. We will discuss how to change the
timeout in the advanced parameters appendix.
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