Linux

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.