Drooling Over Docker #2 — Understanding Union File Systems

Rahul Singh
Oct 7, 2017 · 5 min read

To understand the composition of Container images better, let us take a detour and learn about Union File Systems first.

File Systems and Mounting in Unix/GNU Linux

In Unix (and GNU Linux), everything is a file i.e. — apart from regular data files, even system devices are exposed through a file system name space, e.g. a hard disk can be seen as a file named sda in the directory for device files i.e. /dev and can be accessed through its absolute path being /dev/sda.

Even disk partitions are seen as device files within rootfs e.g. /dev/sda1 is the first primary partition of disk represented by /dev/sda1. Such a partition is further formatted with a file system driver (etx3, ext4 etc.) so that it can store files and directories within it.

Now, if we have a formatted (have a file system) partition /dev/sda1 that has some data stored on it in files organized within a few sub-directories and we want to read from or write to those files — we will have to attach this file system (on file /dev/sda1) to some directory in the logical file system tree. This process is known as mounting (to an existing directory) a file system.

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#mount -t ext4 /dev/sda1 /mnt

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Union File System — what is that!?

The keyword here is UNION as in SET THEORY.

If you experiment with simple mounting feature, and mount two separate file systems at the same mount point one after the other, you’ll only get to see the files from the file system that was mounted last (However, mounting with UNION feature presents contents of all the mounted file systems at the same mount point.).

Let us understand it through a few commands —

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Unlike simple mount option, a union mount would provide a UNION of both the file systems mounted at the same mount point -

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After these examples, now consider that you have a read-only file system and you want to modify a certain file in there so that you can go ahead with your computing needs — on the lines of above mentioned example, Union File System can help us here. We can create another read-write file system either on disk or in RAM as the case may be, and mount both these file systems to another mount point using Union File System. Now, this mount point can give you access to all the files in both ro and rw file systems. In case, you want to modify any of the files residing on the ro file system, Union File System driver would search for that file and perform a CoW (Copy on Write) to make another copy of the file in rw file system that overrides the copy that exists on ro file system. This newly created copy is finally updated with the new contents. Any new files as part of software installation would also go in to the rw files system.

Please check Link1 and Link2 for some very good examples on the points discussed so far. I have also given these links in the resources section in the end of the article.

How Union File Systems help Docker Containers

Docker Containers bring you immutable (unchanging over time) software in the form of layers. During build time, you stack multiple such immutable layers of software to get the desired applications with their dependencies. Many a times there are software layers that override the functionality given by the lower layers in the stack. This is only possible by implementing a Union File System. Also, at run time, you download some additional software within the container that is the newer version of some software; such a case would trigger CoW and have the updated copy of it written in the rw layer of the software stack. Even any newly installed software would settle in this very rwtop layer of the stack. I’ll discuss more about Docker Layers in the next chapter…

Contd…

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Resources:

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