Benefits of Over-Provisioning (OP)
How OP Improves SSD Endurance

To understand why an SSD is configured with over-provisioning and how it benefits the SSD controller, we have to delve into the typical operation of an SSD and the limitations of non-volatile NAND Flash memory.

Each NAND Flash cell has a finite life expectancy, based on its program and erase endurance (P/E), which is characterized during the manufacturing process by the NAND Flash manufacturer because each program or erase function executed on a NAND Flash cell erodes the cell’s capability to reliably store an electrical charge and may therefore threaten data integrity.

However, as NAND geometry migrates from 2D to 3D, NAND endurance will improve, die densities will increase and production costs will decrease, making SSDs more affordable.

To summarize, the three main factors that affect SSD endurance are:

  • NAND Flash program/erase endurance and geometry related read/program/erase complexity (Geometry in this case includes 2-dimensional vs 3-dimensional manufacturing technology)
  • SSD capacity
  • SSD controller capability and efficiency (garbage collection, write amplification, block management, wear-leveling, Error Correcting Code).
How OP Improves SSD Performance

Each NAND Flash memory die is constructed of multiple blocks that contain a further multitude of pages.

NAND Flash can be read and written on a page level but only erased on a block level.

If a single page has to be modified or erased on an already programmed page within a block, then the entire block contents consisting of multiple pages must first be read into a temporary memory, and then erased, before the new block contents can be programmed to the same block address.


References

The only scenario in which a page can be written directly to a block within NAND Flash without this tedious read modify-write cycle is when the page is already in an empty state.

Keeping a large quantity of blocks empty and in reserve via over-provisioning aids in keeping performance consistent, especially in random write scenarios that exhibit the highest Write Amplification Factor (WAF). [1]

  1. JESD219: Solid-State Drive (SSD) Endurance Workloads, JEDEC Committee (http://www.jedec.org/standards-documents/docs/jesd219a). These Client and Enterprise workloads represent a standard for the industry to rate their SSDs and derive the rated TBW supported by their SSDs. Note that your workload could vary and the rated TBW specifications may be above or below your workload’s over time, due to the unique WAF from your application.

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