Quick Answer
The answer is a process holding a deleted file open, which causes the df vs du discrepancy where a deleted file held open by a process still consumes disk space. When a file is deleted but a running process retains an open file descriptor to it, the kernel does not release the inode or its associated data blocks until that descriptor is closed; `df` reads the filesystem superblock and counts those blocks as used, while `du` walks the directory tree and cannot see the unlinked file, so it reports less usage. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of filesystem internals and process management, often appearing as a troubleshooting question where `df -h` shows 100% but `du -sh /` shows only 50%. A common trap is assuming disk corruption or a full inode table, but the real culprit is a zombie file held open by a process like a log writer or database. Remember the mnemonic: “df sees the dead, du sees the directory.”
This XK0-005 practice question tests your understanding of troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A server shows /dev/sda1 mounted at / is 100% full in df -h, but du -sh / shows only 50% usage. What is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The filesystem is corrupted
Why wrong: Corruption would typically cause errors in both df and du.
Hidden files are not counted by du
Why wrong: du counts all files, including hidden, by default.
A process is still holding a deleted file open
Deleted files held open by processes continue to occupy space until the process releases them.
The disk has many hard links
Why wrong: Hard links do not consume extra disk space.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A process is still holding a deleted file open
When a file is deleted but still held open by a running process, the file's inode remains allocated and its disk blocks are not freed until the process closes the file descriptor. The `df` command reports filesystem usage by querying the superblock for total and free blocks, so it still counts the space occupied by the deleted-but-open file. In contrast, `du` traverses the directory tree and sums the sizes of files reachable from the specified path; since the deleted file is no longer linked in the directory, `du` does not include it, leading to the discrepancy.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The filesystem is corrupted
Why it's wrong here
Corruption would typically cause errors in both df and du.
Hidden files are not counted by du
Why it's wrong here
du counts all files, including hidden, by default.
A process is still holding a deleted file open
Why this is correct
Deleted files held open by processes continue to occupy space until the process releases them.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The disk has many hard links
Why it's wrong here
Hard links do not consume extra disk space.
Common exam traps
The trap here is that candidates assume `du` and `df` should always match, and they overlook the classic Linux behavior where a file deleted while still open consumes space invisible to `du` but visible to `df`.
Detailed technical explanation
Under the hood, when a file is unlinked (deleted) but still open, the kernel's VFS layer decrements the link count in the inode but does not release the data blocks until the file descriptor count reaches zero. The `df` command reads the filesystem's superblock, which tracks allocated blocks via the block bitmap, while `du` uses the `stat()` system call on each directory entry; the deleted file has no directory entry, so `du` cannot see it. A real-world scenario is a log file that is rotated or deleted while a daemon (e.g., syslog-ng or Apache) still holds it open, causing the disk to fill up even though no file appears in `ls -la`.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
The correct answer is: A process is still holding a deleted file open — When a file is deleted but still held open by a running process, the file's inode remains allocated and its disk blocks are not freed until the process closes the file descriptor. The `df` command reports filesystem usage by querying the superblock for total and free blocks, so it still counts the space occupied by the deleted-but-open file. In contrast, `du` traverses the directory tree and sums the sizes of files reachable from the specified path; since the deleted file is no longer linked in the directory, `du` does not include it, leading to the discrepancy.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This XK0-005 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the XK0-005 exam.
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