Linux – What does mounting a drive on the /home directory do

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I am running a Rocks Cluster version of CentOS 6.5 and have two internal hard drives installed. When I installed the Rocks OS onto my machine it only made use of one of my two 250GB drives. I have formatted and partitioned the unutilized drive (/dev/sdb1) and would like to make the appropriate changes to my system configuration to make use of it. I was thinking of adding the following line to my /etc/fstab file: 

/dev/sdb1 /home ext4 defaults 0 1

But I have two questions regarding what making this change will do. Will this mean it will be mounted at boot up time by default? Will this mean that there will be 250 additional gigabytes that users can utilize in their own personal home folders? Thanks for any insight you can provide!

Edit: Additionally, what will happen if I never assign a partition to be mounted on directories like /opt, /tmp, /usr… Where do they get disk space from?

Best Answer

When you mount a filesystem, the space on that filesystem becomes accessible to users at the path on which it is mounted. So yes, users will gain access to 250 GB of extra space that is dedicated to /home.

Remember that mounting a filesystem renders any data otherwise located at the mount point inaccessible until it is unmounted, so make sure you move any existing files at /home (without /dev/sdb1 mounted), including permissions, to the new filesystem or users will not be able to access their data. (You might want to use a temporary mount point, like /mnt, to mount /dev/sdb1 so you can transfer files there from the existing /home.)

If no separate filesystem has been mounted for a particular directory or its parent directories, the data in that directory are stored in the root filesystem.

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