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Read redirects when VM vMotions

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When a VM vMotions between hosts either within a cluster or across VMware clusters, VirtuCache has a read redirect feature that allows reads to be redirected to the cache media in the source host (where the VM migrated from) for a period of 10 minutes, before cutting over to the local cache media. So for the first 10 minutes, if the reads are available in the local cache media (of the host the VM migrated to), VirtuCache will obviously read data from cache media of the local host. If the data is not in the local host cache media, and only for the first 10 minutes after the vmotion, VirtuCache will re-direct the read to the cache media of the source host. After 10 minutes VirtuCache completely cuts over to cache media in the destination host, and stops looking up the source host for read requests from the emigrated VM.

The read redirects happen over the replication network. Since VirtuCache knows the IP address of the source ESXi host (from where the VM emigrated from), it redirects the reads to the source host cache media. If the IP connectivity between the destination ESXi and the source ESXi is functional, then VirtuCache would redirect the reads over the replication network which hosts the source and destination IPs. Hence it is critical that VirtuCache Read Re-direct traffic is allowed to go over the Replication network. Towards that end, please ensure that the Replication network does not use the vMotion TCP/IP stack in VMware, which prevents all traffic except VMware’s vMotion traffic from going over this network. Instead, please use VMware’s ‘Default’ TCP/IP stack which will allow all traffic to over the network.

Also, peer connections between VirtuCache instances running on different ESXi hosts that are in the same cluster are not established if these ESXi hosts have different VirtuCache build versions running. Hence all hosts in a cluster should be running the same VirtuCache build. Review this KB article on how to find out the VirtuCache build number.

Although if VirtuCache is running with a Write-Through license only (and not Write-Back license) across the cluster, then it is fine to have VirtuCache installed on some hosts and not others. However, even in this case and for those hosts that have VirtuCache installed, the build version installed on every host should be the same.

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