XK0-006 Troubleshooting Practice Question
A production web server intermittently stops responding for about 30 seconds at a time. The administrator runs 'vmstat 1' during an incident and observes the 'wa' column consistently above 80 while 'us' and 'sy' remain low. Which conclusion best describes the bottleneck?
⚠ Common exam trap
The trap here is reading high wa as a CPU problem, when it actually measures idle time spent waiting for storage to respond.
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
✓
Processes are blocked waiting on I/O, indicating a storage subsystem bottleneck rather than CPU saturation.
In vmstat output the wa column reports the percentage of CPU time spent idle while outstanding I/O requests exist. When wa is very high while us and sy stay low, the processors are not the limiting factor; the storage path is. The intermittent multi-second stalls are consistent with a storage device or array struggling to service requests, so the administrator should investigate disk health, queue depth, and I/O load.
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 kernel is spending excessive time in system calls, indicating a runaway kernel thread.
Why it's wrong here
Excessive time in system calls would appear in the sy column, which the administrator observed as low. The wa column specifically measures idle time waiting on I/O, not kernel CPU consumption. A runaway kernel thread would typically raise sy and possibly us, so this interpretation contradicts the collected data.
- ✓
Processes are blocked waiting on I/O, indicating a storage subsystem bottleneck rather than CPU saturation.
Why this is correct
A high wa value in vmstat means CPUs are idle while waiting for I/O operations to complete. Low us and sy confirm the processors are not the constraint. This points to slow or overloaded storage, such as a failing disk, saturated array, or heavy write load, which matches the intermittent stalls and makes storage the correct diagnosis.
- ✗
User-space applications are consuming all available CPU cycles, indicating a runaway process.
Why it's wrong here
User-space CPU consumption is reported in the us column, which remained low during the incident. High wa with low us means the CPUs were idle waiting for storage, not busy running applications. Concluding a runaway process would require elevated us values and would misdirect the troubleshooting effort.
- ✗
The system is swapping heavily because physical memory is exhausted, indicating a memory shortfall.
Why it's wrong here
Swapping activity is visible through the si and so columns, which measure swap-in and swap-out. While swap can cause I/O wait, the question attributes the stall to high wa without elevated si or so. Diagnosing a pure memory shortfall would need evidence such as high swap rates or an out-of-memory condition, not just elevated wait time.
Go deeper
Related to this question
Learn chapter
Managing Storage and File Systems
Key term
Output
In IT service management, output is the result or deliverable produced by a process, system, or component, such as data, reports, or services delivered to a customer.
Key term
Service
A service is a software component or system that performs a specific function and is available to be used by other programs or users over a network.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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