ESXi hosts can store a boot build and a standby build.

For each update, the update utility updates the standby build. After the update, you then reboot the host. After the host reboots, the standby build becomes the boot build, and the previous boot build becomes the standby build. If the update is successful, the host continues to boot from the new boot build until the next update.

For example, suppose the current boot build is 52252 and the current standby build is 51605. When you update the host to build 52386, the update process replaces build 51605 with build 52386 and makes build 52252 the standby build. If the update is successful, you continue to boot from build 52386 until the next update.

If an update fails and the ESXi 4.0 host cannot boot from the new build, the host reverts to booting from the original boot build.