Having created a skeleton root filesystem in your staging directory, the next task is to transfer it to the target. In the sections that follow, I will describe three possibilities:
- ramdisk: a filesystem image that is loaded into RAM by the bootloader. Ramdisks are easy to create and have no dependencies on mass storage drivers. They can be used in fall-back maintenance mode when the main root filesystem needs updating. They can even be used as the main root filesystem in small embedded devices and, of course, as the early user space in mainstream Linux distributions. A compressed ramdisk uses the minimum amount of storage but still consumes RAM. The contents are volatile so you need another storage type to store permanent data such as configuration parameters.
- disk image: a copy of the root filesystem formatted and ready to be loaded onto a mass storage device on the target. For example, it could be an image in
ext4format ready to be copied onto an SD card, or it could be injffs2format ready to be loaded into flash memory via the bootloader. Creating a disk image is probably the most common option. There is more information about the different types of mass storage in Chapter 7, Creating a Storage Strategy. - network filesystem: the staging directory can be exported to the network via an NFS server and mounted by the target at boot-time. This is often done during the development phase in preference to repeated cycles of creating a disk image and reloading it onto the mass storage device, which is quite a slow process.
I will start with ramdisk and use it to illustrate a few refinements to the root filesystem, like adding user names and a device manager to create device nodes automatically. Then, I will show you how to create a disk image and, finally, how to use NFS to mount the root filesystem over a network.
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