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Inside the system: resources, kernel, processes / Stage 8 of 12

dmesg and the kernel log

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Step 8 of 8 · summary · 2 min

Summary

  • The kernel writes about failures into a ring buffer: sudo dmesg -T reads it, and sudo journalctl -k reads the copy in the journal that survives a reboot.
  • The EXT4-fs error line names the device, the inode and the process; after it ext4 switches writing off, and e2fsck on the unmounted device repairs the filesystem.
  • segfault at … in … is a program crash from touching memory not its own, status 139, and the line's level is info.
  • An OOM report shows who asked for memory, the group's processes and whom the kernel killed.
  • The archive from the start of the lesson answered Bad message and Input/output error with rw in findmnt because the kernel found the damaged inode of orders-07.csv and switched writing off for the whole filesystem.

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Reference for this lesson

Articles

  • Exit codes: $?, && and || — An exit code is a number from 0 to 255 that a command reports: 0 means success, anything else a failure or a "no".
  • The kernel — The core of the operating system: it manages processes, memory, devices, and filesystems; programs request its services through system calls. uname -r prints its version.
  • dmesg — kernel messages: OOM, disk and filesystem errors, segfaults — dmesg prints the kernel ring buffer: messages about disks, filesystems, OOM, crashed programs and hardware.
  • journalctl — reading the systemd journal — Command for reading the systemd journal with service, time, and other filters.
  • OOM killer: when memory runs out — A kernel mechanism: when memory runs out system-wide or in a cgroup with a limit, it picks the process with the highest oom_score and kills it with SIGKILL. Such a process exits with code 137; a container shows OOMKilled: true.
  • Mounting and fstab — Mounting filesystems: the mount command, persistent mounts in /etc/fstab, UUIDs and options.
  • Filesystems: ext4, xfs, fsck — Filesystems: creating one with mkfs, labels and UUIDs, fsck checks, choosing between ext4 and xfs.
Ubuntu 24.04
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