Is fsck -N mounted file system dangerous

user193693 asked:

I’ve read to never run fsck when the file system is mounted. What about running fsck -N to only identify but not repair the file system when mounted? Is that dangerous? Is fsck -N known to overload a system and slow it down?

I answered:

By definition, a mounted filesystem is in an inconsistent state. So if you were to run that, it would likely print a bunch of spurious and completely meaningless errors.

Really, if you need to check the filesystem, unmount it.

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