This LFCS practice question tests your understanding of operation of running systems. 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 developer reports that a web application is running out of file descriptors. Which sysctl parameter should be reviewed and potentially increased?
fs.nr_open
Why wrong: This is the per-process limit, but the system-wide limit is fs.file-max.
fs.file-max
This is the system-wide maximum number of open file descriptors.
kernel.max_files
Why wrong: No such kernel parameter exists.
net.ipv4.tcp_max_syn_backlog
Why wrong: This controls the TCP SYN backlog, not file descriptors.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
fs.file-max
The `fs.file-max` sysctl parameter defines the system-wide limit on the number of open file descriptors that the kernel can allocate. When a web application runs out of file descriptors, it is typically because this global limit has been reached, and increasing it allows more concurrent open files, sockets, and other descriptors. This directly resolves the reported exhaustion.
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.
fs.nr_open
Why it's wrong here
This is the per-process limit, but the system-wide limit is fs.file-max.
fs.file-max
Why this is correct
This is the system-wide maximum number of open file descriptors.
Related concept
Read the scenario before looking for a memorised answer.
kernel.max_files
Why it's wrong here
No such kernel parameter exists.
net.ipv4.tcp_max_syn_backlog
Why it's wrong here
This controls the TCP SYN backlog, not file descriptors.
Common exam traps
The trap here is that candidates confuse the system-wide limit (`fs.file-max`) with the per-process limit (`fs.nr_open` or `ulimit -n`), leading them to choose `fs.nr_open` when the question explicitly asks about a system-wide exhaustion reported by the developer.
Detailed technical explanation
Under the hood, the kernel tracks file descriptors via a global `files_struct` and the `nr_files` counter; when `nr_files` reaches `fs.file-max`, the `open()` system call fails with `ENFILE`. In real-world scenarios, a web server like Nginx or Apache may hit this limit under heavy load, especially if it keeps many persistent connections or logs to many files. Monitoring `/proc/sys/fs/file-nr` shows the current number of allocated file descriptors versus the limit.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
The correct answer is: fs.file-max — The `fs.file-max` sysctl parameter defines the system-wide limit on the number of open file descriptors that the kernel can allocate. When a web application runs out of file descriptors, it is typically because this global limit has been reached, and increasing it allows more concurrent open files, sockets, and other descriptors. This directly resolves the reported exhaustion.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation 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 LFCS exam.
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