hardMultiple ChoiceObjective-mapped
XK0-006 Practice Question: A Linux server is experiencing high I/O wait, as…
A Linux server is experiencing high I/O wait, as shown by the 'iostat -x' command: 'avg-cpu: %iowait=45' and '/dev/sda: await=120ms, %util=95%'. The server has 16 GB of RAM, and the administrator notices that the system is using a significant amount of swap: 'free -m' shows 4 GB of swap used out of 8 GB. The server runs a database application that performs many synchronous writes. The administrator wants to reduce I/O wait without adding physical memory. Which of the following kernel parameter changes is most likely to help by reducing the frequency of write operations to disk?
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
✓
Increase vm.dirty_writeback_centisecs from 500 to 1000.
Increasing vm.dirty_writeback_centisecs from 500 to 1000 halves the frequency of writeback flushes, allowing dirty pages to accumulate longer. This reduces the number of write operations to disk over time, which can lower I/O wait, especially if merges make each write more efficient. While synchronous database writes (fsync) are not directly delayed by this parameter, reducing background flushes frees up I/O bandwidth for those writes, lowering their wait times. Option A (increasing vm.dirty_ratio) allows more dirty pages but does not reduce write frequency; it may even cause larger bursts. Option C (setting vm.swappiness to 0) would not directly affect write frequency or I/O wait from database writes. Option D (decreasing vm.dirty_background_ratio) would start writeback sooner, increasing write frequency and likely worsening I/O wait. Therefore, Option B is correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase vm.dirty_ratio from 20 to 40.
Why it's wrong here
Dirty_ratio sets the maximum percentage of dirty memory before processes are forced to write; increasing it could cause large, bursty writes, potentially increasing iowait.
- ✓
Increase vm.dirty_writeback_centisecs from 500 to 1000.
Why this is correct
Increasing the interval between writeback cycles allows more dirty pages to accumulate, possibly merging writes and reducing the number of I/O operations, thus lowering iowait.
- ✗
Set vm.swappiness to 0.
Why it's wrong here
Swappiness controls swap usage; setting to 0 avoids swapping unless necessary, but the high swap usage indicates memory pressure; this change might not reduce iowait and could even increase it if processes are swapped out less.
- ✗
Decrease vm.dirty_background_ratio from 10 to 5.
Why it's wrong here
Dirty_background_ratio starts writeback earlier; decreasing it would write more frequently, likely increasing iowait.
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Learn chapter
Linux Fundamentals and History
Key term
Kernel
The kernel is the core program of an operating system that manages hardware resources and provides essential services for all other software to run.
Key term
Linux
Linux is an open-source operating system that manages computer hardware and software, widely used in servers, desktops, and embedded systems.
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