What Do You Mean Atomically?

Know your filesystem

Once I worked on a project whose origin predated the widespread use of DNS. They used /etc/hosts files to map host names to IP addresses. This system had grown to thousands of hosts, and the /etc/hosts file was about 1M in size. After any update, which was handled by a sysadmin, they pushed this file to all production hosts.

“Wow! You update those files atomically when you push, right?” I asked the sysadmin.

“What do you mean, atomically?” he asked.

True story!

They wrote the file to an NFS-mounted partition and then copied the file into place on each production host. I was stunned. How did they get away with that sloppy approach? This sysadmin had privilege on the production hosts, yet had no idea how their file systems worked.

I never got an answer to the last question, but have always made sure that everyone I work with understands what atomic operations are, and how to use them in a unix file system.

What is an atomic operation?

An atomic operation is indivisible, in that any outside observer can see either the before state, or the after state, but nothing in between. In other words, it is an all-or-nothing operation.

Atomic operations in the file system

Consider the file-system operations you use from the command line. Which ones are atomic? Of course the answer depends on the file system you use. But unix file systems are broadly similar in this regard.

If you create a new file with touch, is that atomic? If not, what would a partially-created file look like? What about deleting a file with rm? What about cp? Have you ever copied a very large file in background and watched the destination grow in size on disk? What does that say about the atomicity of file copies? What about opening or closing a file? Renaming a file? If you are working on NFS how does that change the answers?

Applying the knowledge

Understanding these basic file-system semantics lets you use the file system as an ad-hoc database. You can provide data to programs without risking an incomplete delivery. You can build processing queues in a directory. You can update your /etc/hosts file knowing that no reader will encounter a malformed file.

Using atomic file-system operations you can avoid a whole class of errors due to incomplete inputs or intermediate results in a data processing sequence. Your systems will be more robust and predictable.

The file system is a ubiquitous, simple, and powerful tool available across unix environments. Learning how it works is well worth the effort. It will make you a better sysadmin and programmer.


What operations are atomic in the traditional unix file system?

How does the behavior of mv vary if there are multiple directories involved? What about multiple file systems?

Can you write a directory-based job queue using atomic updates?

We want to connect with people who enjoy these questions. Tell us what you think.

See also