Wirebook › Glossary › Journaling
What is a journaling file system?
In one sentence: When a file system writes a quick note of what it's about to change before changing it, so it can tidy up after a sudden power cut.
A little more
A file system keeps a list of where every file lives. Saving a file means changing the file itself and that list, and if the power cuts out halfway, the two can disagree. A journaling file system first writes a short note, the journal, saying what it's about to change. Then it makes the change, then marks the note done. After a crash it reads the journal and either finishes the half-done jobs or undoes them, so the list is back in order within seconds.
NTFS on Windows, APFS and Mac OS Extended on Macs, and ext4 on Linux all keep a journal. FAT32 and exFAT, the formats most memory cards and USB sticks use, don't, which is why yanking a stick out mid-copy is more likely to leave a damaged file there. A journal protects the file system's list, not necessarily the file you were saving, so ejecting and backups still matter.
See it in action
- 8Where do deleted files and photos go?Delete a photo, empty the bin, and watch what really happens on a pretend disk: the file is forgotten, not rubbed out.7 min · interactive
- 12How does a game remember where you got to?Play a tiny game, switch the pretend tablet off and see what survives: working memory (RAM) versus the storage drawer, autosave checkpoints, a battery dying mid-save, checksums and cloud saves.6 min · interactive
Do it for real
Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.