John Fremlin:

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MJ Ray <markj@cloaked.freeserve.co.uk> writes:

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Hold AltGr and SysRq, then tap out s u u b (sync, unmount, unmount, boot) You should sync after umount too or else you'll probably get spurious fscks on boot (it needs to write a clean umount flag to the superblock)

Surely a sync won't do anything to a read-only disk? (OK, I lied: "u" is actually remount-read-only.)

Depends whether a remount-read-only flushes out the 'file system clean' bit when it's done at this level, or whether the change waits for the next sync, which is presumably the concern. Aiui a normal remount/umount wouldn't return until the flush is complete, but this method is far from normal.