Multiple GCE instances with one root persistent disk

Question

I finally got into Google Compute Engine today! I am confused about one point and I fear that my gut instinct is correct but I wanted to make sure. I was fooling around with the f1-micro instance because I am cheap and don't want to accidentally wind up with a several hundred dollar bill (thanks Windows Azure!) and these instances do not have a scratch disk, instead you have to create a persistent disk and use it as a root disk to boot off of. That's fine, but when you create the disk automatically it creates a 10GB disk. I read that you can attach more than one instance to a persistent disk but that the disk must be read only mode. Since it doesn't make much sense to have a boot disk be read only, I don't think it's going to be possible to attach multiple instances to a root disk. I tried and it the result was an error. The error was about creating multiple instances of a compute instance so I might have had the command wrong. Does anyone know if it is possible to create a root persistent disk and then attach multiple compute instances to it?

Answer 1

You need a separate root persistent disk for each running instance. You can use snapshots to create additional copies of an existing persistent disk.

Answer 2

In theory you could boot from a read-only device with a small read-write overlayfs that will absorb the writes. This is similar to how you boot a physical host from a USB drive.