There is an application trying to write to disk but complains that there is no space. df shows that there is space. What is the problem?

If df shows that there is free space on the disk, but the application reports no available space, possible reasons include:

• Disk is filled with inodes. Even if there is free space, the number of available inodes (structures for storing file information) may be exhausted, preventing new files from being created.
• Quota restrictions. Quotas may be set at the user or group level for disk usage.
• Access rights issues. The application may not have write permissions to the required directory.
• Mounting of tmpfs or other virtual filesystems. Space may be available on the main disk, but the application writes to another mount point with limited space.

For diagnostics, check:

df -i  # check inode usage
quota -v  # check quotas
mount  # check mount points
ls -ld /path/to/directory  # check permissions