📘 Read Units
- These measure how much data is retrieved from the disk.
- Every time an application or user requests data (like opening a file or querying a database), it consumes read units.
- In cloud databases (like Azure Cosmos DB or AWS DynamoDB), read units are often tied to throughput capacity—how many reads per second you can perform.
✏️ Write Units
- These measure how much data is written or saved to the disk.
- Anytime you create, update, or delete data, write units are consumed.
- Like read units, write units are also used to calculate performance and billing in cloud environments.
🧮 Example (Cloud Database Context)
| Operation | Units Consumed | Description |
| Read 1 KB data | 1 Read Unit | Small query or file access |
| Write 1 KB data | 1 Write Unit | Saving a small record or file |
| Read 4 KB data | 4 Read Units | Larger query or file access |
| Write 4 KB data | 4 Write Units | Writing a larger record or file |
🛠️ In Local Disk Terms
If you’re looking at disk performance on your own computer:
- Read operations = how often your system pulls data from the disk.
- Write operations = how often your system saves or modifies data on the disk.
- These are usually measured in IOPS (Input/Output Operations Per Second), not “units,” but the concept is similar.
⏱️ Power – On Hours
- Definition: The total number of hours the disk has been powered on.
- What it tells you : How long the drive has been active since it was first used.This includes idle time, even if no read / write operations occurred.
- Typical lifespan : Many drives are rated for around 43, 800 hours(5 years of 24 / 7 use).
- Use case : Helps estimate wear and predict potential failure.A drive with 40, 000 + hours might be nearing the end of its expected life—even if it’s still working fine.
🔁 Power Cycle Count
- Definition : The number of times the disk has been powered on and off.
- What it tells you : How often the drive has gone through a full startup / shutdown sequence.
- Why it matters : Frequent power cycles can stress components, especially in HDDs where mechanical parts are involved.SSDs are more resilient but still track this metric.