I had a discussion about this recently – it’s not something I’ve looked at for a while, so thought it worth a post here. When you are configuring a virtual machine’s disks you get presented with a number of options such as Independent Persistent Mode and Independent Non-Persistent Mode. But, what do they mean and, how do they affect the operation of the virtual machine? Let’s have a look at the two modes in more detail:
Independent Persistent Mode Disks
When a virtual machine is configured with a disk in this mode, and you take a snapshot, you will notice that a delta file for the disk(s) in independent persistent mode isn’t created. Writes will continue to go to this disk as though a snapshot isn’t in place, and when that snapshot is removed, the changes to the disk will remain.
Independent Non-Persistent Mode Disks
When a virtual machine is configured with a disk in this mode a redo log is created to contain any subsequent writes to the disk. This means that if the snapshot is removed, or the virtual machine is powered off, then all writes/changes contained in the redo log are discarded.
Why Use Independent Disks?
The main thing to bear in mind with independent disks (in any mode) is that they do no support snapshot operations. This is useful, for example, if you want to exclude certain virtual machine disks from snapshot based backups.
A more detailed example might be that of a public internet PC such as in a library or hotel, where you may wish to reset the state of the virtual machine, discarding any changes, every time the virtual machine is power cycled.
Another example might a content server, perhaps configured with three disks. Disk 1 contains the OS and is set to be dependent. Disk 2 stores the content (which rarely changes) and can be set as Independent Non-Persistent, whilst the 3rd Disk is dedicated to logging and can be set as Independent Persistent so that logs are not lost following snapshot removal. For anyone that has used Citrix Provisioning Server, you could liken the Independent Non-Persistent disk to the write-cache area.