Understand how files, directories, and durability actually work, so data written is data that survives a crash. Use when writing files that matter or debugging corruption and permission problems.
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---
name: file-systems
description: Understand how files, directories, and durability actually work, so data written is data that survives a crash. Use when writing files that matter or debugging corruption and permission problems.
---
# File systems
Writing a file appears simple and involves caching, buffering, and
metadata that determine whether the data survives a power failure. Most
data loss in application code comes from assuming a write means durable.
## Method
1. **Know that writing is not persisting.** Data sits in buffers until
flushed and synced, and a crash before that loses it however
successfully the write returned.
2. **Use atomic replacement for important files.** Write to a temporary
file, sync it, then rename over the target, since rename is atomic
and a partial write is not (see backup-restore).
3. **Sync the directory too when creating files.** The file's data and
the directory entry are separate metadata, and both need durability.
4. **Understand path resolution and symlinks.** A path is resolved at
use time, which is the basis of a whole class of race conditions in
privileged code (see file-upload-safety).
5. **Handle permissions and ownership explicitly.** Files inherit
defaults that differ between environments, which is why permission
bugs appear only in production.
6. **Respect the limits.** Maximum path length, filename character
restrictions, and case sensitivity differ between systems and break
cross-platform code.
7. **Do not use the filesystem as a database.** Concurrent access,
partial writes, and locking are solved problems elsewhere, and
directory-as-index designs fail at scale (see file-storage-design).
## Boundaries
Behaviour differs substantially between filesystems and operating
systems, particularly for durability guarantees. Network and distributed
filesystems break assumptions that hold locally, especially around
locking. Cloud object storage is not a filesystem and has different
consistency semantics (see cloud-storage-selection).
