/dev/mapper

The /dev/mapper/distro--vg-root in place of a traditional /dev/sdxN block device just indicates that you chose to install the system using LVM2 logical volume management.

The devices in /dev/mapper are Logical Volumes. Logical Volume Manager (LVM) uses /dev/mapper and is considered a vastly superior way to manage storage space compared to just creating regular partitions with file systems on various disks.

The subfolders under /dev/mapper are the names of Volume Groups with the device nodes in those subfolders representing Logical Volumes.

cache directory full

with the command

df -h|grep mapper

To find out if it is full. Look for something like:

/dev/mapper/mint--vg-root  101G   61G   36G  64% /

to find out whats filling it up. A big cache file from gimp or something like that

 sudo du -a /home | sort -n -r | head -n 10

Shows the top 10 files taking up the most space.