Consequences of path failover in VirtuCache
Here are the steps that happen in VirtuCache when there is a path fail over event on the ESXi host. A path failover event typically happens in ESXi when the active paths to the storage appliance fail, possibly because of a storage controller failure, network failure on the active paths, etc.
If the path failover happens in ESXi in under 30 seconds, then VirtuCache does not treat the path failover as a failure event , because VMware itself doesn’t treat the path failover as a failure event. So if your VMs are in write-through or write-back caching mode, VirtuCache does not delete the read cache and does not try and sync the write cache to backend storage. So there should be no problems encountered.
However if the path failover in ESXi takes more than 30 seconds, then VMware treats this as a failure event and so VirtuCache also treats it as a failure event. Consequently, VirtuCache deletes the read cache and starts the process of syncing the write cache to backend storage appliance. Deleting read cache is quick. The speed at which VirtuCache syncs write cache to backend storage depends on the speed of the backend storage. The slower the storage, the longer it takes to sync the write cache. However the VMs are not blocked during the time that the write cache is syncing with backend storage. Now if your backend storage appliance is very slow, (possibly now even slower than what it was before because of path failover), then you might experience slow VMs till the write cache is completely flushed to the backend storage, since the process of write cache syncing with the backend storage will put additional burden on the storage appliance and network.
Once the original active path is online, storage traffic should start using this path again (in case of Round Robin PSP), and during the time the traffic flips back to using the original path, there should be no additional problems with VM performance.
if the original path never comes online, here too, there should be no additional VM performance issues, since some other path(s) will be promoted to Active.