Linux: read-only file system (EROFS)

What the message looks like

touch: cannot touch '/var/www/test': Read-only file system

Common causes

• ext4 with errors=remount-ro detected an error and switched to read-only.
• Failing disk or SSD, I/O errors in the VM's storage backend.
• Mounted ro in /etc/fstab, or a deliberately read-only root (containers, appliances).
• systemd hardening: ProtectSystem=strict makes everything except ReadWritePaths read-only for the service.
• Thin-provisioned storage full on the hypervisor.

Check

1. Show mount options of the affected path
   findmnt -o TARGET,SOURCE,FSTYPE,OPTIONS -T /var/wwwCopy
2. Search kernel messages for filesystem and I/O errors
   dmesg -T | grep -iE 'EXT4-fs error|I/O error|remount|read-only' | tail -n 20Copy
3. Check the disk's SMART status
   smartctl -H -A /dev/sdaCopy
4. Show the service's systemd hardening
   systemctl show nginx.service -p ProtectSystem -p ReadWritePaths -p ProtectHomeCopy

Fix

• Back up data first – while it can still be read.
• For filesystem errors run fsck -f on the unmounted device from a rescue system.
• Only if the cause is clear and harmless: mount -o remount,rw /.
• For systemd services allow the needed paths via override: ReadWritePaths=/var/www/uploads.
• With I/O or SMART errors replace the disk or contact the hoster.

FAQ

Can I just run remount,rw?

Technically yes, but if the kernel switched because of errors you are writing to a damaged filesystem. Check first, then remount.

Why can root write but the service cannot?

Then systemd hardening is usually active. ProtectSystem and ReadOnlyPaths only apply in the service's namespace, not in your shell.