Disks fill silently and break everything at once.
WHAT HAPPENS WHEN A DISK FILLS
Applications cannot write. Databases may become read-only or corrupt. Logs stop. Mail stops being delivered. Sessions fail.
The symptoms rarely point at disk space, which makes it a frustrating diagnosis.
CHECKING
df reports space by filesystem. du identifies which directories are largest.
Check inodes separately. A filesystem can exhaust inodes while showing free space, usually from very many small files.
WHAT FILLS DISKS
Log files that are not rotated Backups stored locally Old package versions and caches Session files Email, particularly with attachments Database growth Temporary files from failed operations
PREVENTION
Configure log rotation and confirm it runs Store backups off the server Clean package caches periodically Monitor and alert at eighty per cent
THE EMERGENCY RESPONSE
Clear the largest safe candidates: old logs, package caches, old backups. Then find what actually grew.
MONITORING
Alert on disk usage. It is the single most valuable alert to have.
