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I am new to filesystems and trying to understand mounting.
Is it something like adding a node in tree?

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Way back in the dawn of computing history, storage consisted of paper and then magnetic tape. To connect storage to the computer you would literally hang or mount the tape on the tape reader. I'm sure that, for awhile anyway, after magnetic disks were introduced tapes hung around as file storage -- not merely back up. It was probably easier to reuse the existing terminology (and code) than invent a new concept for disks and thus we continue to mount a disk-based (or even network-based) file system.

Mounting a filesystem, means taking that storage and connecting it to the operating system in a way that it's usable as a hierarchical storage device with directories and files. This could be the initial filesystem (root of the directory hierarchy) or another filesystem that connects to the root filesystem at a mount point, extending the directory hierarchy. In that sense, it is like extending a tree by adding another tree to it.

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    Early hard disk systems consisted of a removable stack of platters that rested in a disk drive the size of a top-loading washing machine. All the electronics were in the drive. You would mount the stack of platters in the disk drive. Drives were expensive and the disks were physically large but low capacity (e.g. 40 MB) so you'd have several sets of platters (i.e. disks) per drive and you'd mount the appropriate disk for the task at hand. Sep 10, 2011 at 17:54
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At the most abstract level, it involves adding the filesystem's information into the VFS such that file accesses are directed to the appropriate place on the filesystem.

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While @tvanfosson gave a good definition of mounting, my answer concerns the entirety of the phrase "mounting a filesystem":

Technically it doesn't mean anything :)

While software engineers may say "mount a filesystem" (e.g. in man mount), it is more precise to say we mount volumes. The word "filesystem" is often used synonymously with "volume" (especially in older documents), but filesystems are actually an abstract concept in computer science; a type of logic.

So to reiterate: a volume is an implementation of a filesystem. You can't say a filesystem resides in a volume either.

Since you can't mount an abstract concept, it doesn't mean anything. Again, I'm aware the common usage implies volume, I'm just being pedantic.

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