71 lines
2.7 KiB
Markdown
71 lines
2.7 KiB
Markdown
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---
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title: Safely reboot a frozen Linux System - SysRq
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date: "2018-02-18"
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categories:
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- misc
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tags:
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- english
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- linux
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summary: Sometimes your Linux is just utterly broken and hung up. This is how you make your system shut down in an orderly fashion to avoid data corruption on your file-system
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cover:
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image: "/images/crashed.jpg"
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showtoc: false
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---
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Until today, I thought the SysRq-Key (like the Scroll Lock Key) is one
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of the useless keys on a modern keyboard. Today I got asked if modern
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operating systems handle the startup triggered by a reboot different
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from a cold boot.
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While I <s>didn't find an answer to the initial
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question</s> ([Wikipedia](https://en.wikipedia.org/wiki/Reboot_(computing)#Cold_vs._warm_reboot)
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to the rescue) I stumbled upon
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[this](https://en.wikipedia.org/wiki/Magic_SysRq_key).
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Basically, the Linux kernel is permanently waiting for special key
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events. Whenever you press Alt + SysRq + B, your machine will
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immediately reboot. This is nice to know, but not really "save" as it
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doesn't gracefully stop the running programs and also doesn't finish
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writing to disk. So we have to tell the Kernel to do exactly this.
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The Wikipedia mentions a nice acronym to remember the keystrokes.
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> **R**eboot **E**ven **I**f **S**ystem **U**tterly **B**roken
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**Slowly** pressing those keys together with Alt + SysRq will trigger
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the following events.
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R — will take keyboard control away from X.
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E — will send a so-called SIGTERM, this is the request the kernel
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sends whenever a program should stop gracefully. You could imagine this
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as a bouncer asking you to leave.
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I — will go one step further, it will send SIGKILL, this signal tells
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the process to just go away, imagine the bouncer violently throwing you
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out. This will close all processes which are hung up or refuse to shut
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down.
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S — will sync all data to disk. Now give it some seconds until you do
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the next step, to make sure it really wrote everything to disk.
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U — will remount all file-systems as read-only. It's not possible to
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unmount all file-systems while running, so this is the closest you'll
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get.
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Now press B which forcefully and immediately reboots.
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If all this didn't work, then either your kernel crashed (at which point
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your system is already shut down) or this feature is disabled in your
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current OS/Kernel release. If you want to enable it, have a look at the
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[ArchWiki](https://wiki.archlinux.org/index.php/Keyboard_shortcuts#Kernel),
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it's a great resource for Linux tutorials even if you don't use an
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ArchLinux derivative.
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## tl; dr
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Pressing Alt + SysRq and then very slowly entering "REISUB" while still
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holding the Alt key will restart your system gracefully. If it doesn't
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work, either the kernel crashed or this functionality is disabled.
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