/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.

The library libdevmapper.so is used for LVM management.

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.