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I am finally getting settled into the intricacies of NIX however one thing I am still trying to figure out is why UNIX assigns hard disk space to each directory in its filesystem.

I discovered this while trying to create a new directory in root where I could put ISO's for running Virtual Machines


As shown below using df command ; some directories have more space than others

e.g. root has only 700mb assigned to it whilst /export/home has 65gb

    /dev/fd            (fd                ):       0 blocks        0 files
/tmp               (swap              ): 7724392 blocks   576352 files
/var/run           (swap              ): 7724392 blocks   576352 files
/export/home       (/dev/dsk/c0d0s7   ):138187048 blocks  8350700 files

In windows there is no such thing like this I believe; each directory takes space that it needs at that time.

What is the advantage of UNIX managing space this way, it seems a bit inflexible in my opinion or perhaps I am just used to windows too much.

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2 Answers 2

up vote 5 down vote accepted

DF doesn't show folders.
It shows the partitions and how they are "mounted" (linked) within the overall filesystem.

First column is the place in the filesystem where the partition attaches.
2nd gives a reference to the partition itself.
And the other columns show additonal info like size used/free.

You are confusing the fixed size of a filesystem and the underlying partition with the notion of a folder within such a filsesystem.

To further muddy the waters: Some of these partitions are virtual and only exist in memory (/proc, sometimes /tmp) and therefore don't relate to any physical partition on disk at all.

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I believe you are mistaken about the meaning of those filesystems. On my Kubuntu machine, df yields the following output (by the way, the option -h prints sizes in human units, rather than blocks):

  $ df -h
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda1        28G  6,9G   20G  27% /
  none            4,0K     0  4,0K   0% /sys/fs/cgroup
  udev            2,9G  4,0K  2,9G   1% /dev
  tmpfs           585M  1,4M  584M   1% /run
  none            5,0M     0  5,0M   0% /run/lock
  none            2,9G  820K  2,9G   1% /run/shm
  none            100M   20K  100M   1% /run/user
  /dev/sda6       202G   96G   97G  50% /home

Your confusion is born out of the fact that what you call finite space directories are not real filesystems on a disk, but are instead Virtual File systems, i.e. parts of the common *Nix filesystem hierarchy, which are hosted directly inside the pc RAM. RAM filesystems are hosted directly in the pc main memory to allow fastest access to them, so that you will find among them /tmp and /proc. They have a definite size because the RAM is limited in its capacity, and because not all RAM space can be allocated to these filesystems. The use of RAM disks is not limited to the OS, but is open also to a (competent) user: you can learn how to generate one such filesystem in in this ccessible Web page. It should also be clear that, under the unfortunate hypothesis that the space allocated in RAM were to become insufficient, the system would expand the virtual filesystem in the swap area.

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You mean swap yes? –  loosebruce Dec 9 '13 at 11:56
@loosebruce You are perfectly right, I have fixed it, thank you. –  MariusMatutiae Dec 9 '13 at 12:03

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