Technical Brief

Understanding Over-provisioning (OP)

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 per cent 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

Applications can be read intensive, such as typical client workloads where a user will generally do 20% writes to 80% reads. Enterprise applications using a storage device for read caching will be read intensive; if these applications write more data to a storage device, then they would be more write-intensive.

The OP capacity set by the SSD manufacturer can vary in size, depending on the application class of the SSD and the total NAND Flash memory capacity.

Larger capacity and different application class drives are typically configured with proportionally bigger over-provisioning due to the resource requirements in managing more NAND Flash with the use of garbage collection, spare blocks and enhanced data protection features.

This OP capacity is non-user accessible and invisible to the host operating system. It is strictly reserved for the SSD controller’s use.

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