why sometimes we need to mount those files under root? [closed]

I am a newbie to mount. From what I know, the root file / is automatically mounted to /dev/xvda1 or /dev/sda1. In some of the tutorials, people mount file like /dummy to /dev/xvdb1, I don’t understand what the meaning of doing this. Because it’s parent root file / has already mounted. Could someone explain this to me? Thanks in advance.

Answer

To throw out a very non-exclusive list of possibilities:

• Sometimes / doesn’t have the capacity for what you intend to use it for, so you want to use a filesystem located on a different physical device for extra storage.
• Sometimes you want to mount content from a filesystem that isn’t capable of being used as root — for instance, a FAT or NTFS filesystem, which doesn’t properly support UNIX semantics.
• Sometimes your other block device is removable, and you’re mounting it only temporarily.
• Sometimes your other block device is located on media that isn’t available at boot time — requiring iSCSI setup or other operations that prevent it from being used as root without initrd / initramfs facilities your operating system doesn’t provide.
• Sometimes you want to use a different filesystem with different semantics — for instance, maybe your xvdb1 is a GFS shared-block filesystem that other machines also have mounted at the same time for combined storage.
• Sometimes you have a read-only block device with bulk contents that can’t change, and you’re mounting it to multiple VMs, vs systems having their own local read-write storage.

The number of possibilities is nearly endless.

This isn’t a software development question, and doesn’t belong on StackOverflow.