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Read-only, escalate on I/O errors

Read-only filesystem on Linux

Check mount options and kernel storage errors before remounting read-write or editing fstab.

Safest first command

findmnt -no TARGET,OPTIONS /

Before you run this

Expected output: The root mount target and mount options, including ro or rw, errors policy, and filesystem-specific flags.

When not to use it: Do not remount read-write before checking kernel I/O errors, filesystem errors, cloud disk state, or storage health.

Expected output example

/ rw,relatime,errors=remount-ro

How to read the result

If options include ro, the filesystem is currently mounted read-only. If options include errors=remount-ro, the kernel may remount read-only after filesystem errors.

What to check next

Mount options show ro

Means: The filesystem is read-only now; writes will fail until the cause is addressed.

Next step: Check kernel logs for I/O or filesystem errors before remounting.

dmesg -T | grep -iE "read-only|I/O error|EXT4-fs error"

Kernel logs show I/O errors

Means: Storage, filesystem, or underlying volume trouble may be active.

Next step: Stop and escalate rather than forcing writes.

Read Current-Boot Logs for One Service

fstab changed recently

Means: Boot or remount options may be wrong.

Next step: Inspect mount definitions and rollback before rebooting.

Inspect the Unit File and Drop-ins Together

Read-only filesystem decision tree

Prove whether the filesystem is mounted read-only, then check kernel and filesystem errors. Remounting rw may hide the symptom while risking data loss if storage is failing.

  1. findmnt -no TARGET,OPTIONS /
  2. mount | grep ' ro,'
  3. dmesg -T | grep -iE 'read-only|I/O error|EXT4-fs error'

fstab and mount branch

If the issue followed a reboot or mount change, inspect fstab and actual mount options together. Do not reboot repeatedly when the root filesystem may have storage errors.

  1. findmnt -no SOURCE,TARGET,FSTYPE,OPTIONS /
  2. cat /etc/fstab

Bad fixes to avoid

Do not run mount -o remount,rw as the first diagnostic step. Do not edit fstab from memory. Do not run filesystem repair on a mounted production filesystem without a recovery plan.

Common causes

  • Filesystem remounted after errors
  • Cloud or virtual disk issue
  • Bad fstab option
  • Underlying block device read-only
  • Storage full or corrupted filesystem

What not to change yet

  • Do not remount rw before checking kernel errors.
  • Do not reboot before preserving logs when remote access may not return.
  • Do not repair a mounted filesystem without a plan.

Stop and escalate if

  • The next step could interrupt users, remove data, or lock out access.
  • The output includes secrets, customer data, or private infrastructure details.
  • You cannot explain the blast radius of the repair command.

platform notes

Distro and service notes

Escalation

I/O errors, filesystem errors, or read-only block devices should be treated as storage incidents.

fstab

A bad fstab change can cause boot failures; keep rollback access.

supporting commands

Command path

Guides and drills