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On my Debian Linux laptop a couple of days ago I got my 23GB root EXT4 partition full (probably I installed more software than I expected).

Since I have plenty of space in my BTRFS home partition, 466GB of which 400GB available, I moved the contents of /usr/share/doc into /home/share/doc, and I created a link /usr/share/doc -> /home/share/doc. The size of this doc directory was around 2.4GB.

After this change, the system has been working correctly for the past 24 hours.

All at once I got the KDE menu bar and the desktop environment disappear, with only the applications' windows still open. I switched to the command line, and with df -h I found out that all the 466GB of my home partition are used.

With tree --du -h . from /home, I get "4.8G used", a realistic figure.

So, who is right? df or tree? Where do the remaining 460GB come from?

Since the doc directory I copied is still 2.4GB, I suspect that moving it and creating the link is not the reason of my problem.

How can I fix this and get a stable system back?


Update:

sudo btrfs filesystem usage -h -T /home

Overall:
    Device size:         465.66GiB
    Device allocated:    465.66GiB
    Device unallocated:      0.00B
    Device missing:          0.00B
    Used:                464.28GiB
    Free (estimated):     64.00KiB  (min: 64.00KiB)
    Data ratio:               1.00
    Metadata ratio:           1.00
    Global reserve:      490.34MiB  (used: 0.00B)

                   Data        Metadata    System
Id Path            single      single      single      Unallocated
-- --------------- ---------- ----------- ----------- ------------
 1 /dev/nvme0n1p9  463.65GiB   2.01GiB      4.00MiB          0.00B
-- --------------- ---------- ----------- ----------- ------------
Total              463.65GiB   2.01GiB      4.00MiB          0.00B
Used               463.65GiB 644.72MiB     80.00KiB          0.00B

Debian 9.5
Dell Precision 7720
home directory: BTRFS
/ directory: EXT4

closed as too broad by Rui F Ribeiro, user88036, Romeo Ninov, Isaac, jimmij Oct 15 '18 at 15:55

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    With BTRFS neither df nor tree are to be trusted. Instead use btrfs fi usage. Since BTRFS supports subvolumes you can actually mount your /home and /usr/share/doc from the same BTRFS filesystem, instead of using a symlink. What does btrfs subvol list /home say? – Emmanuel Rosa Oct 12 '18 at 7:51
  • @EmmanuelRosa - I just added the output of btrfs fi usage in my question. From sudo btrfs subvol list /home I get nothing. – Pietro Oct 12 '18 at 9:20
  • Oh, something wonky happened. There's a better way to utilize your BTRFS filesystem, but the usage issue must be fixed first. It's almost like a process wrote a bunch of data to BTRFS and ended up consuming all of the space. Try using sudo btrfs filesystem du /home to get a list of file sizes. – Emmanuel Rosa Oct 12 '18 at 10:13

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