Q: The `du -sh /` and `df -h` commands show significantly different disk usage values. `df` shows 90% used, but `du` reports only 60% of the disk in use. What explains this discrepancy?

The du and df commands measure disk usage differently:

• du walks the directory tree and sums up the sizes of files it can see (existing directory entries).
• df queries the filesystem's superblock, which tracks total allocated vs. free blocks.
• Deleted files still held open by processes: When a file is deleted (rm) but a process still has it open, du can't see it (directory entry is gone), but df still counts the blocks as used. Fix: lsof +L1 finds files with zero links (deleted but open). Restart the process or truncate via /proc//fd/.

Common causes of discrepancy:

• Reserved blocks: ext4 reserves 5% of the partition for root by default. df accounts for these reserved blocks; du doesn't know about them. Check with tune2fs -l /dev/sda1 | grep "Reserved block count". Reduce if needed: tune2fs -m 1 /dev/sda1 (sets reserved to 1%).
• Filesystem journal: The ext4 journal consumes space visible to df but not du.