why '/' ,"/usr/" and "/var" cant be umounted

[vickey_20]

I went to single user mode to create backup using the dump command. As mentioned in the guide backup can only be made of umounted partition or read only partitions, I tried umounting /usr and /var and / . But was interrupted with device busy error. Though the problem can be solved by remounting the system with readonly , I wonder why we cant unmount it??

[Rubberman]

Besides the fact that /usr and /var are usually found on /, so trying to unmount one will unmount all, it just so happens that /, /usr, and /var are required for the system to function at all. As you found, you can either remount them read-only, or alternatively you can boot with a rescue/live CD/DVD/USB drive and backup that way since the system drives would be unmounted.

[vickey_20]

in my case / and /usr and /var are different partitions, I forgot to mention it

[Rubberman]

Well, it's like I said; /usr and /var are needed to run the system, at least in read-only mode. Myself, I usually boot a rescue CD and backup stuff that way so I am certain that all the files are closed and the system is in a "passive" state since the rescue system runs out of ram disc or off of the CD.