Confused when you see two capacities in the name of a solid state drive? In most cases, the larger number represents the total flash capacity of the drive and the smaller number is the usable storage space on the drive. The reason for this is often because the drive you're looking at has been over-provisioned by the manufacturer.

What is Over Provisioning?

Over Provisioning (OP) is when a certain percentage of an SSD's free space has been allocated to help maximize the lifetime, endurance, and overall performance of the drive itself. It does so by providing a permanent "swap space" for the drive's controller to manage all the data traffic and storage for the drive. By guaranteeing this free space for the controller to move data more efficiently, the SSD no longer has to waste time preparing space on demand for writing, overwriting, and accomplishing basic NAND maintenance tasks like Garbage Collection, Wear-Leveling, and Bad Block Management as the drive fills with data under sustained workloads.

While some users may still see over provisioning as a costly waste of storage capacity, it actually offers significant benefits in smaller capacity drives and drives with heavy application usage. For example, a 64GB SSD at 7% OP would only be left with 60GB of storage space for the user (more examples seen in the chart below) but no longer runs the risk of the drive filling completely, allowing behind the scenes processes to continue running smoothly without interruptions in performance to get the most from the drive during its life span.

Detailed Benefits of Over Provisioning

• Increased Drive Performance - OP improves Random Read/Write performance and IOPS by giving the flash controller additional buffer space for managing program/erase (P/E) cycles and making sure a write operation will have immediate access to a pre-erased block.
   
• Sustained Performance Over Time - OP helps the SSD stays fast even as the drive fills throughout read/write cycles by guaranteeing your controller has the space it needs to perform. 
   
• Increased Drive Endurance - By distributing the total number of writes and erases across a larger population of NAND flash blocks and pages over time, it allows the drive to sustain it's workload.
   
• Prolonged NAND Flash/Drive Life - OP helps your drive work smarter, not harder, resulting in minimized wear and tear on your drive.
   
• SSD Safeguard - Guaranteed "swap space" acts as a safeguard to make sure the user doesn't completely fill the drive to ensure there is sufficient unused capacity available on the SSD and enough idle time to run TRIM, Garbage Collection, and more.
   
• Lowers Power Consumption - OP allows the controller to operate smoothly and, in turn, requires less energy from your device to do its job.
   
• Increased Reliability - As NAND flash fabrication processes become more complex, cramming more flash into smaller chips, OP drives are becoming common practice among manufacturers to guarantee reliability as usage and application expectations rise.
   
• Efficient Wear Leveling - Over Provisioning allows the controller to constantly move data around to ensure that the cells wear evenly and preemptively prepares “free blocks” to use for future data writes, extending the lifespan of each NAND cell.
   
• Reduced time for Garbage Collection - Garbage Collection is a background operation that requires free blocks to copy, consolidate, and temporarily store good data while erasing blocks of invalid data. OP increases the size of the controller's “work bench", giving it extra free space needed to move data or retires worn out cells, resulting in faster execution.

Comparing Full Capacity and Over Provisioned Capacity

This chart illustrates the difference between the amount of flash on the drive and the available storage capacity after over provisioning.

For More on Over Provisioning

We suggest reading the following in-depth articles on the subject:

Exploring the Relationship Between Spare Area and Performance Consistency in Modern SSDs
AnandTech.com

Understanding SSD over-provisioning
EDN.com