[硬件知识]OP（Over-provisioning）预留空间
While solid-state drives (SSDs) are often similar to mechanical Hard Disk Drives (HDDs) in physical dimensions (e.g., height, width and length) and external interface (e.g., SATA or SAS interface), the internal low level operation and components of an SSD differ vastly from the spinning magnetic platter design of an HDD.
After an SSD is assembled, the SSD manufacturer can reserve an additional percentage of the total drive capacity for Over-Provisioning (OP) during firmware programming. Over-provisioning improves performance and often increases the endurance of the SSD, helping the drive last longer due to the SSD Controller having more Flash NAND storage available to alleviate NAND Flash wear over its useful life.
To calculate the over-provisioned (OP) percentage of an SSD, the formula in Figure 1 can be used.
| Percentage Over-provisioning = |
Physical Capacity - User Capacity User Capacity |
Figure 1. Over-provisioning percentage formula.
It is common to see 7 percent overprovisioning present in many SSDs. See Figure 2 for a breakdown of physical capacity present in an SSD versus available user capacity after overprovisioning.
| Physical capacity | User capacity | % Over-Provisioning | Application Class |
|---|---|---|---|
| 64 GB | 60 GB | 7% | Read Intensive |
| 96 GB | 90 GB | 7% | Read Intensive |
| 128 GB | 120 GB | 7% | Read Intensive |
| 128 GB | 100 GB | 28% | More Write Intensive |
| 256 GB | 240 GB | 7% | Read Intensive |
| 256 GB | 200 GB | 28% | More Write Intensive |
| 512 GB | 480 GB | 7% | Read Intensive |
| 512 GB | 400GB | 28% | More Write Intensive |
| 1024GB | 960GB | 7% | Read Intensive |
| 1024GB | 800GB | 28% | More Write Intensive |
| 2048GB | 1800GB | 14% | Read Intensive |
| 2048GB | 1600GB | 28% | More Write Intensive |
Figure 2 Over-provisioning based on capacity and application class
转自：
https://www.kingston.com/us/ssd/overprovisioning
http://blog.csdn.net/guojing3625/article/details/10136551
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