# High disk usage

> Source: Courseiva IT Certification Glossary — https://courseiva.com/glossary/high-disk-usage

## Quick definition

High disk usage means your computer's hard drive or SSD is working very hard, often at 100% of its ability. This can make your computer feel slow, freeze, or take a long time to open files and programs. It is a common problem that can be caused by too many programs running at once or a lack of memory.

## Simple meaning

Imagine you have only one checkout lane open at a grocery store and suddenly a hundred shoppers all try to pay at the same time. The cashier can only process one person at a time, so the line gets long, everyone waits, and the whole process feels painfully slow. High disk usage is very similar inside your computer. Your hard drive or SSD is like that cashier. It has a limited ability to read and write data at any given moment. When you open a program, save a file, or run a background system update, your computer sends requests to the disk to fetch or store information. If too many requests arrive at once, the disk becomes overwhelmed. It cannot keep up, so it queues the requests. You then experience lag, spinning cursors, programs that take forever to open, and a system that feels sluggish or even frozen.

This problem is especially noticeable on older computers with traditional hard drives, but it can also happen on newer machines with SSDs if the system is under heavy strain. Common causes include having too many startup programs, running antivirus scans while doing other work, a virus or malware that is secretly using the disk, or the operating system itself doing background tasks like indexing files for search or downloading updates. When you see 100% disk usage in Task Manager, it is a clear sign that your disk is the bottleneck and cannot handle the current workload. The fix often involves identifying what is hogging the disk and either closing that program, disabling a startup item, or adding more RAM so that the computer does not have to rely so heavily on the disk for temporary storage. Just like opening more checkout lanes can reduce the wait at a grocery store, reducing the number of simultaneous disk requests or upgrading to a faster drive can restore your computer's speed.

## Technical definition

High disk usage refers to a sustained condition where a storage device, either a Hard Disk Drive (HDD) or a Solid-State Drive (SSD), operates at or near its maximum Input/Output Operations Per Second (IOPS) or data transfer capacity, causing system latency and degraded performance. In the context of IT troubleshooting, disk usage is typically measured as a percentage of the disk’s active time over a sampling interval, often displayed in tools like Windows Task Manager, Performance Monitor, or Linux iostat. A reading of 100% indicates that the disk is continuously busy servicing read and write requests, leaving no idle time. This can occur even if the volume of data being transferred is not near the disk’s maximum throughput, because the bottleneck is often the number of concurrent random I/O requests rather than sequential throughput.

From a low-level perspective, an HDD consists of spinning platters and a moving actuator arm with read/write heads. Each random I/O request requires a seek time (the time to move the arm to the correct track) and rotational latency (waiting for the platter to spin the correct sector under the head). These mechanical delays are measured in milliseconds, and when many requests arrive, the queue depth grows. The disk must service these requests sequentially, causing exponential increases in latency. An SSD has no moving parts but has a limited number of NAND flash channels and a controller that manages wear leveling, garbage collection, and error correction. While SSDs have much lower latency than HDDs, they can still saturate their internal queues under heavy workloads, especially with small random writes. High disk usage is often a symptom of an underlying issue rather than a hardware failure, though prolonged high usage can accelerate wear on SSDs.

IT professionals diagnose high disk usage using tools such as Windows Resource Monitor, which shows disk queue length and the specific processes causing I/O. In Linux, commands like iostat, iotop, and atop provide per-process disk usage statistics. Common software causes include the Windows Superfetch (SysMain) service, Windows Search indexing, antivirus real-time scanning, memory paging (when the system runs out of RAM and uses the disk as virtual memory), and misconfigured backup software. On servers, high disk usage may be caused by a database transaction log growth, a runaway process, or a failing disk that causes excessive retries. Temporary fixes involve disabling non-essential services, adjusting power settings to prevent disk spin-down, and using tools like Process Explorer to identify the culprit. Long-term solutions include upgrading to an SSD, adding more RAM to reduce paging, or migrating to a storage tier with higher IOPS. In exam contexts, understanding the difference between high usage caused by paging versus a malware infection is a key troubleshooting skill.

## Real-life example

Think about a popular food truck that serves amazing tacos. It is lunchtime, and the truck has only one cook and one small grill. The cook can grill about 10 tacos every 5 minutes. Suddenly, 50 hungry people line up at the same time. Each person wants 2 tacos. The cook can only cook 10 tacos every 5 minutes no matter how fast she moves, so the line grows. People wait 25 minutes or more. That is exactly what happens during high disk usage. The disk (the cook) can only handle a certain number of read and write requests (tacos) per second. When the operating system and programs (the hungry customers) all send requests at once, the disk gets overwhelmed. Requests pile up in a queue (the line). Instead of completing a read in 10 milliseconds, a read might take 200 milliseconds or more. The whole system feels slow because every action from opening a file to launching a browser now has to wait in the line.

Now, suppose the food truck could add a second cook and a larger grill. That would be like upgrading from a slow HDD to a faster SSD or adding more memory so the system does not have to use the disk as much. Suddenly, the truck can serve 30 tacos in 5 minutes. The line moves faster, and customers are happier. In your computer, this translates to a faster, more responsive system. The analogy also helps explain why sometimes high disk usage happens even when you are not doing anything. Your computer might be running a background antivirus scan or downloading a system update. That is like the cook getting an huge order from a delivery app that takes up all her time, while the regular customers are left waiting. Until that big order is done, everyone else is stuck. That is why when you see 100% disk usage, you should check what is using the disk and decide if that activity is truly necessary right now. Temporarily pausing a backup or defragmentation can immediately free up the disk and restore system speed, just like asking the food truck to close the delivery app for a while so lunch customers can be served faster.

## Why it matters

For IT professionals, high disk usage is one of the most common performance complaints from end users. It directly affects productivity because it makes computers slow, unresponsive, and frustrating to use. Users often mistake high disk usage for a virus or a failing hard drive, when in reality it might be a simple misconfiguration or a resource-hungry background process. Knowing how to diagnose and resolve high disk usage quickly is a foundational troubleshooting skill for help desk technicians, system administrators, and support engineers. The ability to open Task Manager, identify the offending process, and take corrective action can resolve a ticket in minutes rather than hours. This saves time, reduces user frustration, and builds trust in IT support.

Beyond immediate troubleshooting, high disk usage can be a symptom of larger infrastructure issues. On servers, sustained high disk usage can indicate that the server is undersized for its workload, that a database needs indexing, or that a backup window conflicts with peak usage times. Ignoring high disk usage on an SSD can reduce its lifespan because SSDs have a finite number of write cycles. For enterprise systems, monitoring disk usage trends is part of capacity planning. If a server consistently runs at 90% disk usage, it is a signal that an upgrade or workload rebalancing is needed before it becomes a critical failure point.

Understanding high disk usage also ties into fundamental IT concepts like memory management, I/O scheduling, and system resource monitoring. It reinforces the relationship between RAM, paging, and disk performance. When a system runs out of physical RAM, it uses the disk as swap space, which is exponentially slower than RAM. This causes disk usage to spike. Therefore, adding RAM is a common fix for high disk usage caused by paging. High disk usage is not just a performance nuisance; it is a diagnostic window into the health and configuration of a computer. Mastery of this topic is expected at the CompTIA A+, Network+, and Microsoft MD-100 exam levels, where troubleshooting performance issues is a key objective.

## Why it matters in exams

High disk usage is a core troubleshooting topic in several major IT certification exams, most notably CompTIA A+ (220-1101 and 220-1102), CompTIA Network+ (N10-008), and Microsoft Modern Desktop Administrator Associate (MD-100). In CompTIA A+, the troubleshooting methodology for Windows performance issues explicitly includes identifying high disk usage as a step. Exam objectives under “Troubleshooting PC and mobile device issues” require candidates to know how to use Task Manager, Resource Monitor, and Performance Monitor to diagnose the cause. Questions often present a scenario where a computer is slow, and the candidate must determine whether the issue is high CPU, high RAM, or high disk usage, and then select the correct tool or fix. For example, a question might describe a user with a slow computer and a 100% disk usage reading in Task Manager. The correct answer could be to disable SysMain (Superfetch) or check for a Windows Search indexing issue.

In the Microsoft MD-100 exam, which covers Windows 10 and Windows 11 administration, high disk usage appears under “Troubleshoot performance issues.” Candidates must understand how to use tools like Performance Monitor to create data collector sets and analyze disk performance counters such as “% Disk Time,” “Avg. Disk Queue Length,” and “Current Disk Queue Length.” A common exam trap involves the Windows Search Indexer. The exam expects you to know that rebuilding the index or pausing indexing can reduce disk usage temporarily. In Network+, high disk usage is less directly tested but appears in the context of server performance and network storage. A question might describe a server running slowly, and you need to consider whether disk I/O is the bottleneck before checking network latency.

For general IT certifications, high disk usage is also indirectly tested in scenarios involving virtual memory, paging file configuration, and hardware upgrades. You might be asked to recommend an upgrade for a system that consistently has high disk usage. The correct answer is to add more RAM or upgrade to an SSD. The exams also test your ability to differentiate between high disk usage caused by a virus versus a legitimate system process. For instance, an antimalware scan may cause temporary high disk usage, which is expected, whereas a cryptocurrency miner might cause constant high disk usage. Understanding these distinctions helps you choose the right troubleshooting step, such as running an antivirus scan versus simply waiting for the scan to finish. In all these exams, the ability to read a performance graph, interpret a disk queue length, and correlate it with user symptoms is a skill that is directly tested, making high disk usage a must-know topic.

## How it appears in exam questions

Exam questions about high disk usage typically follow a troubleshooting scenario format. A common pattern is: “A user reports that their computer is very slow, especially when opening files and applications. You open Task Manager and see that disk usage is at 100% while CPU and memory usage are normal. What is the most likely cause?” This type of question tests your ability to recognize that high disk usage can occur independently of other resources. The distractor answers might include upgrading the CPU, increasing the amount of RAM, or disabling the firewall. The correct answer often involves disabling a non-essential system service like SysMain (Superfetch) or adjusting the paging file.

Another frequent question type involves configuration. For example: “A system administrator notices that a server’s disk queue length is consistently above 2 for an extended period. What does this indicate?” Here, the candidate must understand that a queue length persistently above 2 per physical disk suggests that the disk is a bottleneck and cannot keep up with the I/O requests. The correct answer would be to upgrade to a faster disk or add more spindles in a RAID array.

Troubleshooting questions also appear where you must select the correct tool to diagnose high disk usage. For instance: “Which Windows tool would you use to see which specific process is causing high disk usage in real time?” The answer is Resource Monitor (perfmon.exe /res), not just Task Manager alone. Another variant: “A user’s laptop is slow after waking from sleep. Task Manager shows 100% disk usage. What is the first step to resolve this?” The correct answer is to check for and disable any startup programs that might be causing a burst of disk activity upon resume.

Scenario-based questions sometimes include logs or event viewer entries. For example: “You see multiple event ID 129 warnings in the System log indicating a timeout on a disk controller. The user reports intermittent freezing. Disk usage spikes to 100% during these events. What is the most likely cause?” This tests the candidate’s knowledge that such events often indicate a failing disk or a driver issue. The answer could be to check the SATA cable connection or run a disk health diagnostic like CHKDSK or S.M.A.R.T. analysis. Overall, expect high disk usage questions to be integrated into broader performance troubleshooting scenarios, requiring you to identify the symptom, choose the right diagnostic tool, and select the appropriate fix.

## Example scenario

You are a help desk technician at a small company. A user named Sarah calls you and says her computer has been running very slowly all morning. She tells you that she has only one browser window open with three tabs, and she is just trying to edit a Word document. She says the computer freezes for several seconds whenever she tries to save the document, and the mouse cursor keeps spinning. You ask her to press Ctrl+Alt+Delete and open Task Manager. She tells you that the “Disk” column shows 100%, but the CPU and Memory usage are both under 30%. You recognize this as a classic case of high disk usage.

You ask Sarah to check the “Processes” tab in Task Manager to see which program is using the disk the most. She sees that “Microsoft Windows Search Indexer” is using 70% of the disk, “System” is using 20%, and the Word document is using only 3%. You explain that the Windows Search Indexer is a background service that builds a search index of all files on the computer. Sometimes, especially after a new file installation or a system update, the indexer runs aggressively and can saturate the disk. You ask Sarah to wait for the indexing to complete, but she says she needs to finish her work now. You then instruct her to pause the indexing temporarily by going to the Search settings and turning off search indexing for now. She does so, and within a minute, the disk usage drops to 20%, the Word document becomes responsive, and she can save her work without freezing.

You also advise her to schedule the indexing to run overnight when she is not using the computer. You note in the ticket that the computer is an older model with a traditional hard disk drive and only 4 GB of RAM. You recommend upgrading the RAM to 8 GB and consider replacing the HDD with an SSD in the near future to prevent this from happening again. This scenario illustrates how high disk usage can be caused by predictable system processes and how a simple reconfiguration can restore productivity. It also shows the importance of using Task Manager to identify the process responsible, a skill that is directly tested in CompTIA A+ and Microsoft MD-100 exams.

## Common mistakes

- **Mistake:** Assuming high disk usage is always caused by a virus or malware.
  - Why it is wrong: While malware can cause high disk usage, it is not the most common cause. Legitimate system processes like Windows Search Indexer, Superfetch (SysMain), Windows Update, or a scheduled antivirus scan are much more frequent causes on healthy systems.
  - Fix: Always check Task Manager first for the process using the disk before jumping to malware conclusions. If the process is a legitimate system service, investigate whether it is running abnormally before running an antivirus scan.
- **Mistake:** Thinking that 100% disk usage always means the disk is failing.
  - Why it is wrong: 100% disk usage is a performance metric that indicates the disk is busy, not necessarily that it is broken. A failing disk can cause high usage, but it is just as likely, if not more so, to be caused by software issues like a runaway process or insufficient RAM.
  - Fix: Check the disk error event logs and S.M.A.R.T. status for signs of hardware failure. If the disk passes health checks, focus on software and configuration troubleshooting.
- **Mistake:** Attempting to fix high disk usage by upgrading the CPU or adding more RAM without first diagnosing the cause.
  - Why it is wrong: If the high disk usage is caused by a specific software bug or misconfiguration, adding RAM will not help. For example, if the Windows Search Indexer is stuck in an infinite loop, it will continue to use the disk heavily regardless of how much RAM is installed. Upgrading hardware should be a last resort after software causes are eliminated.
  - Fix: Use Resource Monitor to identify the exact process causing high disk usage. If it is a system service, try disabling or restarting it first. Only consider hardware upgrades if the disk usage is consistently high due to legitimate workload demands.
- **Mistake:** Rebooting the computer every time disk usage is high, without documenting the root cause.
  - Why it is wrong: A temporary reboot might clear the disk queue and restore performance, but if the underlying cause (e.g., a scheduled task, a memory leak, or a misconfigured service) is not addressed, the high disk usage will soon return. Rebooting without investigation is a band-aid, not a fix.
  - Fix: After a reboot, monitor the disk usage over the next hour. Check the Event Viewer for repeating errors. If the high usage pattern recurs, investigate the process that consistently appears at the top of the disk usage list.

## Exam trap

{"trap":"When a question shows 100% disk usage and you see that the System process is the top consumer, many candidates assume it is a driver issue or a malware infection and select an answer involving a clean boot or antivirus scan.","why_learners_choose_it":"The “System” process sounds vague and suspicious. Learners often believe that a legitimate system process should not be using so much disk, so they conclude it must be corrupted or infected.","how_to_avoid_it":"Remember that the System process can legitimately cause high disk usage due to memory paging. If the system has insufficient RAM, the System process (specifically the memory manager) will use the disk heavily as virtual memory. Always check the RAM usage first. If RAM is near 100%, the high disk usage is likely due to paging, not malware. The correct fix is to add more RAM, not to run antivirus scans."}

## Commonly confused with

- **High disk usage vs High CPU usage:** High CPU usage means the processor is working hard, often due to heavy calculations or a runaway process. High disk usage means the storage drive is the bottleneck. A system can have high CPU but low disk usage if the CPU is waiting for data from memory. Conversely, high disk usage with low CPU is common when the disk queue is full and the processor is idle waiting for I/O. The key diagnostic difference is which resource shows 100% in Task Manager. (Example: If you are running a video render, CPU might be 100% while disk is low. If you are copying many small files, disk might be 100% while CPU is low.)
- **High disk usage vs High memory usage:** High memory usage occurs when most of the RAM is in use. This can cause high disk usage indirectly because the operating system uses the disk as virtual memory (swap space) when RAM is full. However, high disk usage can also occur without high RAM usage if a process is doing intense file reading or writing. Resource Monitor helps separate the two: if high disk usage is accompanied by high hard fault rates (paging), it is memory-related. If not, it is likely direct disk I/O. (Example: A system with 4 GB RAM and 8 GB of data cached might show 80% memory usage but low disk usage. But if you open 20 browser tabs, memory might hit 100%, causing paging and then 100% disk usage.)
- **High disk usage vs Disk fragmentation:** Disk fragmentation is a condition where file data is scattered across the physical disk, causing slower read/write times because the drive head must move more. High disk usage is a measure of how busy the disk is right now. A heavily fragmented disk can cause high disk usage indirectly because the disk must work harder to piece together files, but the two are not the same. Fragmentation is a long-term condition, while high disk usage is a real-time performance metric. (Example: A fragmented HDD might take 10 seconds to open a file, causing disk usage to spike to 100% during that time. But after defragmentation, the same file opens in 2 seconds, and disk usage stays below 20%.)

## Step-by-step breakdown

1. **Observation and symptom confirmation** — A user reports a slow computer, frequent freezes, or delays when opening files. The technician confirms the symptom by opening Task Manager (Ctrl+Shift+Esc) and navigating to the Performance tab. If the Disk activity shows a sustained 90-100%, the symptom is validated. This step is critical because other issues like network lag or high CPU can mimic similar user complaints.
2. **Identify the offending process** — In the Processes tab of Task Manager, sort the processes by the Disk column. Note which process is using the highest percentage of disk time. Common culprits include SearchIndexer.exe (Windows Search), sysmain.exe (Superfetch), msiexec.exe (Windows Installer), or svchost.exe (various system services). This step pinpoints the software component causing the bottleneck.
3. **Analyze resource correlation** — Check the Memory column to see if RAM usage is near 100%. If it is, the high disk usage is likely due to paging. Also check the Disk Queue Length in the Performance tab (more than 2 per physical disk suggests a bottleneck). This step distinguishes between direct disk I/O issues and indirect memory-induced disk usage.
4. **Apply a temporary fix** — Based on the identified process, apply a temporary solution. If SearchIndexer.exe is the cause, pause indexing from the Search settings. If sysmain.exe is the cause, disable the SysMain service temporarily. If a specific application like an antivirus is scanning, wait for the scan to finish or reschedule it. This step restores immediate productivity while a permanent fix is determined.
5. **Address the root cause** — After the immediate symptom is relieved, implement a permanent solution. If the root cause was insufficient RAM, recommend a memory upgrade. If it was a misconfigured service, adjust its startup type or schedule. If the disk is old or failing, recommend an upgrade to an SSD. This step ensures the problem does not recur.
6. **Monitor and document** — After applying the fix, monitor the disk usage over the next few days. Use Performance Monitor to log disk counters if needed. Document the issue, the process identified, and the resolution in a ticket. This builds a knowledge base for future similar issues and helps in capacity planning.

## Practical mini-lesson

When a technician encounters high disk usage in a production environment, the first priority is to quickly determine whether the issue is a temporary bottleneck or a chronic problem requiring a hardware upgrade. In a help desk context, time is valuable, so a structured approach is essential. Open Task Manager and look at the Disk column. If you see a process like SearchIndexer.exe consuming 50% or more, that is almost always the cause. The Windows Search Indexer rebuilds its database when files are added, modified, or after a major update. On an HDD, this can saturate the disk for hours. The immediate fix is to pause indexing via the Settings app (Privacy & Security > Searching Windows > Exclude relevant folders). This buys time, but you should also ensure that the indexer is not stuck in an endless loop due to corruption. If the indexer consistently causes high disk usage, try rebuilding the index (Control Panel > Indexing Options > Advanced > Rebuild).

If the culprit is sysmain.exe (formerly Superfetch), this service preloads frequently used applications into memory to speed up launches. On systems with limited RAM, it can ironically cause high disk usage as it constantly reads from the disk. Disabling SysMain in Services.msc can resolve this, but be aware that it will increase application launch times. Another frequent cause is the Windows Update service (wuauserv). When Windows downloads and installs updates, it can cause high disk usage, especially on older hardware. The manual workaround is to pause updates for a few days, but ultimately the system needs to update.

On servers, high disk usage often stems from database transaction logs growing unchecked or backup software running during peak hours. An administrator can use PerfMon to track the “Avg. Disk Queue Length” counter. If it consistently exceeds the number of physical spindles in a RAID array, the disk subsystem is the bottleneck. Solutions include adding faster drives (e.g., NVMe SSDs), increasing the number of drives in a RAID 10 array to distribute load, or adjusting the backup window. In virtualized environments, high disk usage on a host can be caused by the “thundering herd” effect when many virtual machines boot simultaneously after a power outage. Staggering startup times or using storage QoS policies can mitigate this.

Professionals also need to understand the difference between high disk usage and high disk latency. High usage indicates the disk is busy; high latency indicates each request is taking a long time. A disk can be at 100% usage but with low latency if the requests are quick. That is less problematic. High latency with moderate usage often indicates a failing disk or a misconfigured queue. Tools like CrystalDiskInfo can check S.M.A.R.T. data for reallocated sectors or pending errors, which signal impending failure. If a disk is failing, the immediate action is to back up data and replace the drive. Practical management of high disk usage requires a mix of real-time monitoring, service configuration, and sometimes hardware upgrades. Always rule out software causes before replacing hardware, and always document the pattern of usage to inform future infrastructure decisions.

## Memory tip

To remember disk usage causes: “S-S-W”, Search indexer, SysMain, Windows Update, the three most common software triggers. If RAM is full, think “Pages to disk = high disk.”

## FAQ

**Is high disk usage dangerous for my computer?**

It is not immediately dangerous, but sustained high disk usage can lead to data corruption if the system crashes while writing. On SSDs, constant high usage reduces the lifespan by wearing out write cycles. It is also a symptom of an underlying issue that should be resolved.

**Can a slow internet connection cause high disk usage?**

No, a slow internet connection does not directly cause high disk usage. However, a web browser that is downloading many files or streaming video may write temporary data to the disk, which could contribute to disk activity, but the internet speed itself is not the cause.

**Why does high disk usage happen after waking from sleep?**

When a computer wakes from sleep, many background processes resume simultaneously. Antivirus may start a quick scan, Windows may check for updates, and indexers may rebuild. This burst of activity can temporarily saturate the disk, especially on slower HDDs.

**Will adding more RAM always fix high disk usage?**

No. Adding more RAM helps only if the high disk usage is caused by insufficient memory forcing the system to use the disk as virtual memory (paging). If the high disk usage is caused by a different process, such as an antivirus scan or Windows Search indexing, adding RAM will not help.

**How can I tell if high disk usage is due to a failing hard drive?**

Use a tool like CrystalDiskInfo or the built-in CHKDSK to check the S.M.A.R.T. status. Look for reallocated sector counts, pending sectors, or high read error rates. If these are abnormal, the disk may be failing. If your computer makes clicking or grinding noises, that is a strong indicator of hardware failure.

**What is the difference between disk usage and disk queue length?**

Disk usage is the percentage of time the disk is busy processing requests. Disk queue length is the number of requests waiting to be processed. A high queue length (over 2 per physical disk) indicates that the disk is a bottleneck, even if disk usage is not at 100% yet.

## Summary

High disk usage is a common performance issue where a computer's storage drive is operating at its maximum capacity, causing slow response times and system freezes. It is a core troubleshooting topic for IT certification exams like CompTIA A+, Microsoft MD-100, and others. The primary causes are often software-related, including the Windows Search Indexer, the SysMain (Superfetch) service, Windows Update, or memory paging due to insufficient RAM. Hardware causes are less common but include failing hard drives or outdated HDDs.

Diagnosing high disk usage involves using tools like Task Manager and Resource Monitor to identify the process responsible. The fix depends on the root cause: disabling or reconfiguring a service, adding more RAM, or upgrading to an SSD. It is important not to confuse high disk usage with high CPU or memory usage, as each requires a different troubleshooting approach. In exams, scenarios often ask you to interpret performance data from Task Manager and choose the correct corrective action, such as disabling SysMain, pausing indexing, or upgrading hardware.

The key takeaway for IT professionals and exam candidates is that high disk usage is rarely a mystery if you follow a structured diagnostic process. Always start by identifying the process, correlate it with memory usage, and then apply targeted fixes. Avoid common mistakes like jumping to malware conclusions or rebooting without investigating the root cause. By mastering this topic, you will be prepared for exam questions and real-world support situations alike.

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Practice questions and the full interactive page: https://courseiva.com/glossary/high-disk-usage