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Over-Provisioning
In brief
Over-provisioning is a reserve area in the flash memory of an SSD that the controller uses internally and to which the operating system and user have no direct access. It serves to distribute write operations, replace defective blocks and improve performance and service life. It matters for data erasure because classic overwriting does not reliably reach this area.
How over-provisioning works
An SSD contains more flash memory than it reports to the system as capacity. Part of the difference already results from the units: flash chips are manufactured in binary sizes (GiB), while capacity is stated in decimal sizes (GB), which creates roughly 7% reserve. Manufacturers can provide additional reserve, often significantly more for server SSDs. The controller uses this area for Wear leveling, background garbage collection and as a replacement for worn blocks.
Significance for secure erasure
If software writes data from outside across the entire reported capacity, the controller decides where it ends up. Old data can remain in reserve blocks in the process. Safer are methods that the controller itself executes and that also cover these areas, such as the Sanitize command or a Crypto Erase. Which method suits which SSD is explained in our guide Securely erasing hard drives and SSDs. Second IT erases SSDs with Blancco and documents every device with an erasure certificate; data carriers that cannot be erased are destroyed in-house.
Frequently asked questions about over-provisioning
Can I enlarge the over-provisioning area myself?
Yes, by leaving part of the SSD unpartitioned or by using manufacturer tools. This can improve performance and service life but reduces the usable capacity.
Is old data really stored in the reserve area?
That is possible. Blocks that the controller has replaced or set aside may still contain earlier content until they are erased.
Is TRIM enough to empty the reserve area?
No. TRIM only tells the SSD which blocks are free. When and whether they are erased is controlled by the controller.
Why does an SSD report less capacity than it has flash memory?
An SSD contains more flash memory than it reports to the system. Part of this results from the units: flash chips are manufactured in binary sizes (GiB), while capacity is stated in decimal sizes (GB). This creates roughly 7% reserve. Manufacturers can provide additional reserve.
How is the over-provisioning area securely erased?
With methods that the controller itself executes and that also cover reserve blocks, such as the Sanitize command or a Crypto Erase. Overwriting from outside is not enough, because the controller decides where the data ends up. Old data can remain in reserve blocks.
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