The last upgrade I worked on was going from 5.1 to 6.5, where we upgraded to 5.5 first, then jumped straight to 6.5. This was in order, doing the vCenter first, then the hosts using the VUM on the newly migrated/upgraded vCenter Server. This next environment is currently running Windows vCenter Server 6.0.0 (6.0U2) with the hosts we have running at 5.1.0 (5.1 Patch 9).
The plan here is to get everything to 6.5, which means migrating and upgrading this vCenter Server to the appliance, as well, then upgrading the hosts to match. It seems better to take a different approach where rather than going from 5.1 to 5.5 to 6.5, we'll go from 5.1 to 6.0 to 6.5, considering the vCenter is already at 6.0.0.
Another difference between these environments is that these hosts are currently running their hypervisors from an SDHC card. We bought 32GB cards, but a teammate went out of order and manually upgraded one of the 2 hosts we have to 6.5 independently on a 32GB card. When learning of the premature jump, we decided to remount the 4GB SD card with the 5.1 system into the machineso we could pull the host back into the cluster and do the vCenter upgrade first, and use the VUM to perform the upgrade of the hosts.
The concern here is SD card space. Will the 4GB card be sufficient? For how long? Can it be upgraded somehow to a bigger card later without significant issues? I understand the minimum requirement is a 1GB card for ESXi 6.5, but what are best practices for upgrading, especially when using VUM from the VCSA (assuming it was upgraded successfully) and a 4GB card already loaded with 5.1? It's a rather tiny environment with only 2 hosts at this time, and about 7 running VMs (will be 8, with the VCSA completed). I've run into issues with ramdisk when upgrading on local datastores, and I've never done this with a host running ESXi from an SD card, so I would like to have some expectations and requirements, given the above information. Thanks, in advance.