Overprovisioning refers to when you leave some unallocated space on your partition table. If you have a 128GB SSD and you create a 110GB NTFS partition and leave 18GB unallocated then you have overprovisioned the SSD by 18GB.
By not creating a partition or applying a file system the controller can use this space as additional spare area. The more spare area you have, the lower the write amplification will be which means your NAND will last longer. It also improves I/O consistency and reduces a slow down if the drive is worked hard.
Samsung's Magician has actively pushed customers to leave some unallocated space on their drive for overprovisioning. Rumour is that some SSDs can use any partitioned space as additional spare area as long as it is unused by the file system.
All this is a balancing act. Drives come with about 7% spare area by default. A light duty machine would not require any additional spare area. An extreme workload machine would benefit hugely from more spare area. You have to decide where your workload fits in. Samsung's Magician recommends to overprovision 19GB for a 128GB SSD. I haven't used any other size than 128GB with Magician so I cannot confirm if that scales up and down accordingly to size, eg 38GB for a 256GB SSD.
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