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How can the root directory be at 100 percent disc usage while its subdirectories not be? Is space allocated on a per directory basis? In the below example, it shows that the root is 100 percent used. Does this mean if I try to add content to one of the nonroot directories, it should work but if I try to add to the root, it won't work? I was expecting that 100 percent usage would be the total of ALL the directories added up, but as the example below shows, that's not the case:

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
                      3.5G  3.3G     0 100% /
none                  1.5G  180K  1.5G   1% /dev
none                  1.5G     0  1.5G   0% /dev/shm
none                  1.5G   40K  1.5G   1% /var/run
none                  1.5G     0  1.5G   0% /var/lock
none                  1.5G     0  1.5G   0% /lib/init/rw
none                  3.5G  3.3G     0 100% /var/lib/ureadahead/debugfs
/dev/sdb               50G   27G   21G  57% /home
/dev/sda1             228M   35M  181M  17% /boot

Thanks for some explanation of disc usage on filesystem.

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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:.

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Linux works a bit differently then windows. You have a "root filesystem", and that is the disk you have mounted (pointed towards) the directory called /. But other discs, like for instance your your /home disc can be there. Where in windows they would be for instance c:\ and d:\ , you end up with them in "subdirectories" in linux, called "mountpoints".

So while the disc that you use as / might be full, if you mount an empty terabyte disc as /home it will be that: empty.

This also means that if you have enough space on /home you cannot add a file to /otherdir: that gets added to an actual subdir on the / partition!

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As df shows, /dev/mapper/eubuntu10x32-root is mounted on / (the root of your file system).

The other "folders" that df shows are not really folders but also what is called "mount point".

For instance,

  • the real partition /dev/mapper/eubuntu10x32-root mounted on / is full.

  • the real partition /dev/sdb mounted on /home is not full (57%).

The thing is that you can't access these disk partitions directly. So partitions have to be mounted on some mount points (folders). For example, the /home folder is called a mount point for /dev/sdb since the physical disk is "linked" to this folder.

This means that when you write something into /home, your data are actually written to /dev/sdb, not /dev/mapper/eubuntu10x32-root. 3.5G is small for a root partition and you lack space on it but you can still write in your home directory because there are 21G available in it.

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