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I recently upgraded one of my set-up to Ubuntu 16 from Ubuntu 14. On doing df -h I am seeing output like below

udev                          7.9G     0  7.9G   0% /dev
tmpfs                         1.6G  171M  1.5G  11% /run
/dev/mapper/vg-root            29G  3.5G   24G  13% /
tmpfs                         7.9G   36K  7.9G   1% /dev/shm
tmpfs                         5.0M     0  5.0M   0% /run/lock
tmpfs                         7.9G     0  7.9G   0% /sys/fs/cgroup
none                          7.9G     0  7.9G   0% /run/shm
/dev/sda1                     1.9G   58M  1.7G   4% /boot
/dev/mapper/vg-tmp             25G  484M   23G   3% /tmp
/dev/mapper/vg-home            30G  6.9G   21G  25% /home
/dev/mapper/vg-var            852G  1.2G  808G   1% /var
/dev/mapper/vg-var+log         49G  258M   46G   1% /var/log
/dev/mapper/vg-var+log+audit   20G   60M   19G   1% /var/log/audit
tmpfs                         1.6G     0  1.6G   0% /run/user/122
tmpfs                         1.6G     0  1.6G   0% /run/user/999
tmpfs                         1.6G     0  1.6G   0% /run/user/118
tmpfs                         1.6G     0  1.6G   0% /run/user/119
tmpfs                         1.6G     0  1.6G   0% /run/user/123
tmpfs                         1.6G     0  1.6G   0% /run/user/0
tmpfs                         1.6G     0  1.6G   0% /run/user/1001

Can someone let me know what does tmpfs file system denote on the above output? I did not see this on Ubuntu 14.

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tmpfs allows allows you to create filesystems in virtual memory (RAM). Just like you can create filesystems on your HD or a USB key and mount it, e.g: /dev/sda1, /dev/sda2/ etc you can also do the same using virtual memory (RAM) but when doing so in Virtual Memory, the contents will be lost once your system is rebooted.

So in your example the /xx/xx/xx references are just paths to these mount points in virtual memory. You don't have to worry about these, they are created and managed automatically by the system.

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Those directories /run/user/$uid were introduced with systemd (Ubuntu 15.04+). From What is this folder /run/user/1000?:

/run/user/$uid is created by pam_systemd and used for storing files used by running processes for that user. These might be things such as your keyring daemon, pulseaudio, etc.

So they are kind of temporary directories private to the corresponding user.

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