Operating-system level virtualisation
As we explained in part 2 of this series, A brief history of virtualisation, in the 1960s, it was a sound move to run one OS on top of a totally different one. On the hardware of the time, full multi-user time-sharing was a big challenge, which virtualisation neatly sidestepped by splitting a tough problem into two smaller, easier ones.
Within a decade, though, a new generation of hardware made it easy enough that a skunkworks project at AT&T was able to create a relatively small, simple OS that was nonetheless a full multi-user, time-sharing one: Unix.
After its early years as a research project, Unix spent a few decades as a proprietary product, with dozens of competing companies offering their own versions – meaning that it splintered into many incompatible varieties. Each company implemented its own enhancements and then its rivals would copy it to create their own versions.
Fairly late in this process, a new form of virtualisation emerged as one of these features. First implemented as part of FreeBSD 4 in 2000, a version came to Linux in 2001, Sun Solaris in 2005 and IBM AIX in 2007. Each Unix calls it by a different name and has slightly different functionality, but the overall concept is the same.
It springs from a far simpler piece of Unix functionality – the humble chroot command, which dates back all the way to Version 7 Unix in 1979. For those Windows-only types out there, a tiny bit of Unix background is needed at this point.
One big directory tree
In all flavours of Unix, there is just one, system-spanning directory tree, starting at the root directory. On CP/M and DOS and Windows, the base level of storage is an assortment of drive letters, on each of which is a directory tree.
Unix does things the other way round: there’s one big directory tree, starting at the root directory – called just “/” – and disk partitions and volumes appear as directories within it.
What chroot lets you do is transform a subdirectory into the root filesystem just for one particular process. You build a skeletal Unix filesystem containing only whatever files are necessary for that process to run, then imprison that process – and any subprocesses it might create – within it, so that it can no longer see the whole directory tree, just its particular subtree.
In essence, then, it virtualises just the filesystem: it doesn't protect the system against a program with superuser – ie, administrative – rights, but it is very handy for testing purposes.
The chroot command proved to be such a useful tool that in time it was extended into a more complete form, which also virtualised the memory space, I/O and so on of the operating system. The result is that a process locked inside the virtual environment seemed to have the whole computer to itself.
To understand how this works, consider how modern operating systems work. In most processors, there are at least two privilege levels at which code can execute, which are generally called something like kernel space and user space.
Code running in kernel space – usually the OS kernel and any essential device drivers – is in direct control of the hardware and can directly manipulate peripherals and so on. In contrast, code running in user space can’t – it just gets given its own block of memory, which is all it’s allowed to access, and it has to ask the kernel nicely for I/O. On x86 chips, kernel code runs at a level called Ring 0 and user code in Ring 3, and the levels in between are left unused.