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What is the magic sysreq combination? (piefed.blahaj.zone)

submitted 1 week ago* (last edited 1 week ago) by sem@piefed.blahaj.zone to c/linux@programming.dev

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Buried in the Wikipedia article is what I remember, with a note saying not to use it any more.

So what sysreq keys do you use to recover a non responsive system with a journaling file system?

Before the advent of journaled filesystems a common use of the magic SysRq key was to perform a safe reboot of a locked-up Linux computer (using the sequence of key presses indicated by the mnemonic REISUB), which lessened the risk of filesystem corruption. With modern filesystems, syncing and unmounting is still useful to force unflushed data to disk, but is no longer necessary to prevent filesystem corruption (and may increase the risk of corruption in case the lock-up is caused by the kernel being in a bad state).^[11]^ The default value of kernel.sysrq in distributions such as Ubuntu and Debian remains 176^[citation needed]^ (allowing the sync, unmount, and reboot functions) and 438^[12]^ (allowing the same functions plus loglevel, unraw, and nice-all-RT-tasks) respectively.

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[–] Telorand@reddthat.com 13 points 1 week ago* (last edited 1 week ago) (1 children)

The clue is in the directions: REISUB

Hold Alt + Print Scr
Then type R E I S U B slowly.
- Give a 2-3sec pause between presses to be sure.

You can look up what each letter does, as each one performs a different function when combined with Alt + Print Scr.

ETA: I seem to recall that you can enter the sequence quicker than that, as I believe I used to, but it's probably good to give your system time to perform each function, especially if you're trying to rescue it.

[–] sukhmel@programming.dev 3 points 1 week ago (1 children)

For me all of those key presses produce dmesg logs that just state all of those are disabled

[–] BCsven@lemmy.ca 2 points 6 days ago* (last edited 6 days ago)

Hmm, I have a vague recollection of a setting to allow sysreq key combos or not.

Edit: Ahh, Rentlar notes it in their comment.

You should check ahead of time whether the value of /proc/sys/kernel/sysrq is not zero to be sure the Sysrq functions are enabled. If not, then in /etc/sysctl.d/99-sysctl.conf you can add the line kernel.sysrq = 1 to enable all features, or a higher number to enable select features.