File systems and mount points

You're missing out on a very important way Ubuntu/Linux handles file systems: all comes down to the same root (/) and other file systems ("volumes", "disks", "partitions") will be mounted on a location inside the root.

To explain two most interesting lines:

• Your / is full:

  
  
  

  /dev/mapper/eubuntu10x32-root   3.5G  3.3G     0 100% /
  
  

  
  
  

  Here you see a "device mapper" meta-device being used for your root filesystem. I think you're using LVM here or encryption.

  
  
  

  3.5GiB for a / is a very low capacity for a desktop installation, unless you have directories like /usr, /var moved out of it. In your case it's clearly not sufficient.




• Your second disk (sdb, without partitions) is mounted at /home and a partition of sda is mounted as /boot:

  
  
  

  /dev/sdb               50G   27G   21G  57% /home
  
  /dev/sda1             228M   35M  181M  17% /boot
  
  

  
  
  

  This separated /boot is common for a LVM/encrypted installation to allow the bootloader (Grub) to load the relevant software/drivers to be able to reach your other file systems to actually boot the operating system.

So, basically, your /home and /boot aren't just directories. They are directories on the / file system, but there's another file system mounted on top of it. This means that whenever you descend into it, you're looking into a different file system. Each file system has its own capacity, listed by df.

While you moved out /home and /boot, this wasn't enough to have sufficient space available on /. Solutions: grow the / file system, move out more like /usr or /var to another file system.

Are all directories under / mount points?

Directories not being a mount point are actual regular directories part of file system in mounted as /. There are also special ones, which don't actually represent data on disks, but in UNIX/Linux, everything is represented as files such as sockets, kernel interfaces (/proc, /sys), etc. and even directories itself are files.

How does this compare to Windows?

Windows tends to hide the difference between the devices/partitions and the actual file system being mounted. This means that a C: "drive" can also be mounted as such. In Ubuntu/Linux, you'll see that a partition, e.g. /dev/sda1 (first partition on sda) can be mounted (content being made available to the user) to any location. There's no second root for this like Windows makes the second drive a D:.