A mounted filesystem must be detached from the directory tree at once, even though processes still hold files open on it, with the real cleanup happening as soon as those references are gone. Which command does that?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard medium
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The options
Correct umount -l /mnt/data
Correct. -l is a lazy unmount: the mount point is detached from the hierarchy immediately, so nothing new can reach it, and the filesystem is released once the last open reference is closed.
Not correct umount -f /mnt/data
Wrong for this description. -f forces an unmount, which exists mainly for an unreachable NFS server; it can abandon in-flight I/O rather than waiting for references to be released.
Not correct umount -r /mnt/data
Wrong. -r means that if the unmount fails, umount should try to remount the filesystem read-only instead. It gives up on unmounting rather than achieving it.
Not correct umount -a
Wrong scope. -a attempts to unmount everything listed in /etc/mtab, and it would still fail on this busy mount for the same reason as a plain umount.
Why
A filesystem is busy while any process holds a file open on it or has a working directory inside it, and a plain umount refuses with 'target is busy'. The three escapes differ in what they sacrifice: -l detaches the name now and frees the filesystem later, -f abandons pending operations, -r downgrades to read-only. Lazy unmounting is the usual choice on a local disk because it never discards writes, but the device stays in use until the last holder exits, so it is not a way to pull a disk out immediately.
Where this comes from
- Cited
- manual page umount(8)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 104 and space the ones you get wrong.
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