When a Linux server suddenly switches to a read-only file system, application failures usually cascade.
In this situation, the first thing to check is not application logs, but kernel and filesystem logs.

---

Representative Kernel Log Patterns (Root Cause) #

EXT4-fs error (device sda1): ext4_journal_check_start: Detected aborted journal
EXT4-fs (sda1): Remounting filesystem read-only

Meaning:
An ext4 journal error has occurred → the kernel forcibly remounts the filesystem as read-only to protect data.

---

Buffer I/O error on dev sda, logical block 123456
blk_update_request: I/O error, dev sda, sector 789012

Meaning:
Disk I/O errors → a high likelihood of physical disk failure or issues in the storage / hypervisor layer.

---

Symptoms Seen in Application Logs (Result) #

touch: cannot touch '/var/run/app.pid': Read-only file system
mkdir: cannot create directory ‘/var/log/app’: Read-only file system

Meaning:
The filesystem is already in a read-only state.
(Note: these logs are results, not the root cause.)

---

Decisive Signals in dmesg #

Aborting journal on device sda1.
EXT4-fs error (device sda1)

If these logs appear → the kernel has completed the decision to block writes.
(This state does not recover automatically without a reboot.)

---

Immediate Check Points (Commands) #

# 1. Check current mount status (verify ro option)
mount | grep ' ro,'

# 2. Search for error patterns in kernel messages
dmesg | egrep -i 'ext4|i/o error|read-only'

# 3. Check recent kernel logs
journalctl -k | tail

---

Key Summary #

• Switching to read-only mode is a protective action, not the root cause.
• The root cause is usually one of the following:
  • Disk I/O errors
  • Filesystem journal corruption
  • Storage subsystem failures
• If Remounting filesystem read-only appears in the logs, the failure has already occurred.

Identifying this pattern first and focusing on the infrastructure / OS layer prevents unnecessary application-level debugging.

🛠 마지막 수정일: 2025.12.23