Intern Learns the Horrifying Truth About File I/O Buffering

Description

This is a meme using the 'Scared boy holding a cross' format. A young boy with a distressed expression holds up a wooden cross and raises his other hand defensively, as if to ward off something evil or unholy. The top text, in a bold, white, all-caps font, reads: 'THE INTERN THAT SPEND HALF DAY CODING FILE ACCESS'. The bottom text reads: 'WHEN YOU START EXPLAINING THAT EVEN AFTER THE FILE IS SUCCESSFULLY CLOSED THE DATA MAY BE STILL ONLY IN KERNEL BUFFER AND NOT FULLY WRITTEN TO PERSISTENT STORAGE'. The meme humorously captures the shock junior developers experience when they move from the simple abstraction of file operations to the complex reality of how operating systems handle I/O. The idea that a successful 'close' operation doesn't guarantee data persistence due to kernel-level buffering is a non-intuitive and often frightening concept for those new to systems programming, as it implies a risk of data loss they hadn't considered. The post's caption, 'The next step is understanding what happens when Ceph or another NFS is used', adds to the joke by suggesting this is just the first layer of a much deeper, more complex world of distributed storage

Common Tags

OperatingSystems DeveloperHumor Filesystem DataConsistency DataPersistence

Comments

Anonymous ★ Top Pick The intern thinks `fclose()` is a transaction commit. The senior knows it's just an optimistic async call and is already typing `fsync()` before the intern's code even compiles

Anonymous ★ Top Pick

The intern thinks `fclose()` is a transaction commit. The senior knows it's just an optimistic async call and is already typing `fsync()` before the intern's code even compiles
Anonymous

Intern: “close() returned 0 - we’re good!” Senior: “Right… after ext4’s journal checkpoint, the drive’s write-back cache flush, and Ceph’s min_size=3 quorum blesses your bytes; until then it’s just cached optimism.”
Anonymous

The real horror isn't teaching interns about file I/O - it's explaining why their perfectly working code still lost data in production because nobody told them about the unholy trinity of fsync(), O_DIRECT, and write barriers. Next week's lesson: why databases spend 90% of their code dealing with what happens when the power cord gets yanked
Anonymous

Ah yes, the classic intern initiation: discovering that close() is merely a polite suggestion to the kernel, not a legally binding contract with your SSD. Nothing says 'welcome to systems programming' quite like explaining that their meticulously crafted file writes are currently enjoying a leisurely vacation in page cache, blissfully unaware they might never make it to persistent storage if the power goes out. It's the moment every senior engineer remembers - when you realize the OS has been gaslighting you about I/O completion this whole time, and fsync() becomes your new best friend and performance bottleneck
Anonymous

close() just drops your fd; durability is an opt-in API with a p99 surcharge called fsync
Anonymous

Explaining to the intern that close() isn’t durability: you still fsync the file, rename, fsync the directory, then hope the SSD’s write cache isn’t lying - eventual consistency on a single machine
Anonymous

Interns celebrate fclose(); veterans mutter 'msync or perish' under their breath
@sevos 2y

Which NFS do you have in mind? Underground? Most wanted?
1. @RiedleroD 2y
  
  I think hot pursuit
  
  @mihanizzm 2y
  
  The man of culture
@casKd_dev 2y

O_DIRECT
@qtsmolcat 2y

We call that "someone else's problem" around here
@Assarbad 2y

Well, where's the followup meme were the same intern started flushing to disk religiously and destroys application performance?
@Mikle_Bond 2y

And even after that it may be living in hard drive's write buffer...
@callofvoid0 2y

wtf
1. @underbermensch 2y
  
  take a look on screenshots, there are yellow filter on the game