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As written in the previous posts, in the ESXi installation there are two copy of the system image located in two different partitions (see Partitions layout of system disk and More on partitions posts).
The actual system image is located on the first 250 MB partition, formatted with plain old FAT16 filesystem, but using a special pseudo-filesystem on it (see VisorFS: A Special-purpose File System for Efficient Handling of System Images). The image itself, s.v00, is a 124 MB compressed file, which is decompressed on boot and contains the hypervisor operating system. Note that the compressed image is larger in 5.0 than in 4.1, (where the image was 70 MB), but still a very small disk foot print with about 124 MB in 5.0.
The entire ESXi works on a RAM-disk and changes in the configuration are not immediately written on the banks. There is an autobackup is a script that runs each hour. You can see the command by simple watch at the /var/spool/cron/crontabs/root file:
#min hour day mon dow command 1 * * * * /sbin/auto-backup.sh
As you can see it simple calls the /sbin/backup.sh script to write the current state on the bank (and you can force the operation manually by simple execute this script).
As written in the previous post about the root password (how to reset of the root password) all the configurations files are in the state file /bootbank/state.tgz that contain a local.tgz file with the configuration.
If you later update the active image it will first be copied to the alternative boot bank, as a “last known good configuration”. If the update should fail you could press SHIFT + R early in the boot sequence and select to boot the system from the alternative boot bank.