Virtual SAN is always a popular topic in our webinar series, and during last week’s What’s New in VMware Virtual SAN 6.2 Senior Technical Instructor Javier Menendez answered a lot of questions from the audience. Here is a small sampling, and you can hear the rest in the recording.
Q: If one needs HA Failover, then do we need at least four ESXi hosts in that case?
A: Virtual SAN should be combined with HA even with three hosts. With three hosts, if a host crashes, the VMs affected will be restarted by HA in a different node. The reason why four hosts is better than three is simple. If a four hosts exists, not only you have more flexibility but besides that, the data lost in one of the hosts can be rebuilt on a fourth host.Q: How does Virtual SAN deal with split brain?
A: With Virtual SANs, that is the role of a witness. If a VM is a mirror, it has two submirrors and a witness. You need majority to continue to operate. Aka, if host1 can’t see host2 and these are the two with the two submirrors of a VM, the one that can access the witness first wins.Q: Is there a performance hit using thin provisioning on the swap files?
A: Not a good idea to use thin provision if memory is overcommitted. The virtual swap files always exist if the VM is running but if you don’t overcommit memory, you are not swapping; hence not writing to the swap object or reading from them.Q: If you have a 3 host ESx Virtual SAN can you have a failure of one host and still keep the VM guest running?
A: In a three node cluster, assuming that a virtual machine is using the default policy (FTT=1; aka a 2-way mirror), the VM will either continue to run or be restarted by HA depending on which host crashes. For example, if it was running on the first server, it will continue to run (no downtime) if the second or third host fails. If the first one fails, the virtual machine will crash since it was using CPU and RAM from that host, but then HA will restart it in either the second or third since the VM was redundant.Q: I am new to Virtual SAN, I would like to clarify if I need an SSD to create a Virtual SAN or can I use only magnetic disks?
A: Flash drives are always needed. Either for caching purposes only (the hybrid model) or both cache and capacity (all-flash model).Q: Can Virtual SAN “intelligently” migrate high IOP machines to flash while keeping less demanding VMs on slower disks?
A: All the machines reside in the capacity tier, while the cache tier is used to improve the performance of all of the virtual machines.Q: Can ESXi hosts be boot-to-SAN and use SAN LUNs for the storage presented to the cluster?
A: The idea is to use ESXi hosts with local drives for Virtual SAN; not LUNs on a physical storage area network.
