Debugging Disk space

To find top 10 files eating up disk space
-- du -kh |sort -nr |head -10

To find disk space per folder/file in current directory.
-- du -sch .[!.]* * |sort -h

To find deleted files still being accessed by any processes
-- lsof +L1

If df looks fine, yet no files can be added, check inodes.
-- df -i

Number of inodes used can be high, if you've several small files.

When your df -h output goes to next multiple lines and you need some awk filters on positions,

Use df -P. It'll print one liners.

Other lesser used:
-- ncdu

-- du β€”max-depth=1 -h /