I'm aware that ESXi maintains two copies of its boot partition, /bootbank and /altbootbank, and that /altbootbank is more or less a backup copy of /bootbank, which is the running copy.

What I'm not clear on, and haven't ever seen documented is:

  1. When /altbootbank is used instead of /bootbank
  2. Whether /bootbank is overwritten or if booting is just done from /altbootbank when it's deemed necessary
  3. Under what circumstances /altbootbank might be updated (i.e. is it always a "factory reset" copy of /bootbank or might it be refreshed - presumably by copying from /bootbank - under some set of circumstances, other than manual intervention)

I would assume that the answer to (1) is "when booting from /bootbank results in an error", but what would happen here? Would the user see any evidence that this had occurred, or need to intervene?

Can anyone enlighten me on one or more of these points? Is there a document somewhere that explains it all?

1 Answer 1



The ESXi system has two independent banks of memory, each of which stores a full system image, as a fail-safe for applying updates. When you upgrade the system, the new version is loaded into the inactive bank of memory, and the system is set to use the updated bank when it reboots. If any problem is detected during the boot process, the system automatically boots from the previously used bank of memory. You can also intervene manually at boot time to choose which image to use for that boot, so you can back out of an update if necessary.

  • 2
    Thanks very much - that document is pretty much what I was hoping for. If anyone finding this is interested, the "live" bootbank is selected based on the value of the updated variable in boot.cfg (this file is read from both partitions and the one with the higher value is chosen). Shift-r during boot manually selects the one with the lower value instead. Everything else is in ynguldyn's doc. Jun 10, 2011 at 9:56
  • This link is now broken - is there an update? Edit: See vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/… Jul 15, 2021 at 2:42

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