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    "Here, you see examples of four system calls - open, read, write and close. Don't fret over the details of the syntax; that's not important right now. The point is this: through these system calls (and a few others) the Linux kernel provides the illusion of a 'file' - a sequence of bytes of data that has a name - and protects you from the underlying details of tracks and sectors and heads and free block lists that you'd have to get into if you wanted to talk to the hardware directly. That's what we mean by an abstraction.
    "As you'll see from the picture above, the kernel has to work hard to maintain this same abstraction when the filesystem itself might be stored in any of several formats, on local storage devices such as hard disks, CDs or USB memory sticks - or might even be on a remote system and accessed through a network protocol such as NFS or CIFS.

    "There may even be an additional device mapper layer to support logical volumes or RAID. The virtual filesystem layer within the kernel enables it to present these underlying forms of storage as a collection of files within a single hierarchical filesystem."