LFCS Operation of Running Systems Practice Question
A systems administrator is troubleshooting a server that runs a database application. The server has 64 GB of RAM and 16 CPU cores. The administrator notices that the system is using a significant amount of swap space even though there is plenty of free memory. The 'free -m' command shows: total memory = 65536, used = 50000, free = 15536, buffers/cache = 10000, swap total = 8192, swap used = 6000. Which of the following is the most likely cause?
⚠ Common exam trap
Linux Foundation often tests the misconception that swap usage only occurs when memory is full, but the trap here is that vm.swappiness can cause swapping even with abundant free memory, leading candidates to overlook the kernel's proactive swapping behavior.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The vm.swappiness value is set too high.
A high vm.swappiness value (default 60) causes the kernel to aggressively swap out anonymous pages even when ample free memory exists. With 15 GB free and 10 GB in buffers/cache, the system should not be using 6 GB of swap unless swappiness is set too high, forcing premature swapping.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The vm.dirty_ratio and vm.dirty_background_ratio are set too high.
Why it's wrong here
Dirty ratios govern how much modified page-cache data may accumulate before writeback; they influence flush timing, not whether anonymous pages get reclaimed to swap. It is tempting because both are vm tunables, but the swap usage with free memory indicates an aggressive swappiness setting instead.
- ✓
The vm.swappiness value is set too high.
Why this is correct
A high vm.swappiness value makes the kernel aggressively reclaim anonymous pages to swap even when free memory remains, matching the observed 6 GB swap usage alongside 15 GB free. Lowering swappiness keeps pages resident until memory pressure genuinely demands swapping.
- ✗
The database is configured to use huge pages, which are not swappable.
Why it's wrong here
Huge pages are locked and non-swappable, so their presence would reduce swap consumption, not cause 6 GB of swap use. It is tempting because huge pages affect memory behaviour, but the observed swapping with free RAM points to swappiness or reclaim pressure, not page-size configuration.
- ✗
The vm.vfs_cache_pressure is set too low.
Why it's wrong here
vfs_cache_pressure controls reclaim of dentry and inode caches; lowering it retains those caches longer and does not push anonymous pages into swap. It is tempting because it is a reclaim-related tunable, but the symptom reflects swappiness driving anonymous-page eviction despite available RAM.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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