Maximize SSD Lifetime and Performance With Over-Provisioning
Over-Provisioning (OP), the practice of allocating a specific, permanent amount of free space on an SSD, is a widely-used method for improving both SSD Performance and Endurance. Historically, Samsung has not implemented mandatory OP on any of its SSDs. With the introduction of the 840 Series and the reality of increasingly complex NAND fabrication processes, however, Samsung has chosen to implement a minimum amount of OP in its mainstream drives (the 840 PRO will not feature mandatory OP).
What is OP?
An SSD controller is responsible for the massive task of managing all data traffic and storage for the drive. NAND technology’s intrinsic complexities require a lot of extra work behind the scenes. A data write is not as simple as placing data into an empty memory bank. Each NAND cell has a limited lifespan – it can only endure a specific number of data reads/writes. An additional layer of complexity is added by the fact that overwriting old data on NAND requires an erase of the entire NAND block (this same block may contain other data that is still valid). As a result, the controller is constantly moving data around to ensure that the cells wear evenly and to preemptively prepare “free blocks” to use for future data writes.
All of this management work requires the SSD to have a kind of “swap space” available to use as temporary storage while the controller goes about its business. The controller will use any available free space for this, but free space becomes a premium commodity as we fill our drives with data. OP is a way to set aside a minimum amount of free space, inaccessible to the user or the OS, which the SSD controller can utilize as a kind of “work bench.”
How do I set OP?
Samsung Magician can assist users in setting up OP on their SSDs. Magician’s simple, graphical OP tool will recommend an optimum amount of OP space (for a consumer workload) and then configure the SSD with just one click. This decision can always be altered or reversed at a later time.
Some drives, however, like most SandForce-driven SSDs and the new SSD 840 Series, set aside a permanent amount of OP. This minimum amount of OP is not user-configurable and will vary from vendor to vendor. There is no “right” amount of OP to set aside, and it is best to vary it by capacity and situation.
If you don’t want to use Magician’s recommendation, or if you are a professional or more advanced user with different requirements, you are free to set aside your own OP space. To decide how much to set aside, you must consider your specific usage patterns as well as the size of your SSD. Users with large SSDs (250GB+) will likely not use all of the available space on their drives as long as they are not storing large amounts of photos, video or other media files. Thus, a casual user with a large-capacity SSD may not need to set aside any extra space for OP. The SSD will naturally use any available free space to perform its maintenance algorithms. If you have a small SSD, on the other hand, it is recommended to set aside some OP (between 6.7 and 10% of total drive space) to minimize the risk of accidentally filling the drive to capacity. While filling a drive with data isn’t harmful, it will have a severe impact on performance. Users with smaller drives are much more likely to fill them with data, effectively taking away the SSD controller’s working space and making it more difficult to prepare free blocks as well as accomplish basic maintenance tasks.
Also, different OP ratio is recommended by usage applications and workload. One might wonder why the recommendations should be variable. Data access patterns are the determining factor. Activities that produce frequent Read/Write requests, especially Random Read/Writes, put extra stress on the SSD, which in turn increases Write Amplification (a phenomenon by which physical NAND writes outnumber logical write requests from the host). Heavy workloads wear out NAND cells faster, which increases the need for Bad Block management so that the controller can retire worn out cells. Retiring a NAND cell requires the controller to copy all of the cell’s valid data to a new block, taken from a limited number of “reserved” blocks. Without empty blocks ready to accommodate this process, performance will suffer. Working in tandem with bad block management, and reducing the need to use it altogether, is Wear-Leveling, which ensures that no single cell is written to more than others. This process also requires data to be copied from one block to another, which further increases the necessity for free blocks to be available. To ensure free blocks are available, Garbage Collection algorithms consolidate good data and erase blocks of invalid data. This process too, however, requires free blocks to use for temporary storage while the invalid blocks are cleared.
OP helps with garbage collection, wear-leveling and bad block management by effectively increasing the size of the controller’s “work bench,” thus giving it extra free space to use while it consolidates and moves data or retires worn out cells.
All of the above tasks are the reason WAF exists – writing data to an SSD is not a simple one-to-one operation. There are complex processes taking place behind the scenes at all times.
Why Use OP?
OP has a direct effect on SSD performance under sustained workloads and as the drive is filled with data. Guaranteeing free space to accomplish the NAND management tasks discussed above (Garbage Collection, Wear-Leveling, Bad Block Management) means the SSD does not have to waste time preparing space on demand, a process that requires additional time as data is copied, erased, and recopied. An added benefit is that OP makes all of the SSD maintenance procedures more efficient, reducing the WAF by ensuring there’s room to work. Consider this scenario: You are a chef with very limited counter space (say one arm’s length across and half and arm’s length deep). Preparing a 5-course meal in such a limited area would mean you have to waste a lot of time moving things around or putting them away to make room for other tasks. Now, imagine you quadruple your work space. You can leave everything you need at hand, reduce repeating steps (like getting the salt from the pantry and putting it back away), and increase your working speed. The same goes for SSDs. Give them more room to work, and they can do it more quickly and efficiently, reducing WAF and increasing performance at the same time.
OP and the 840 Series SSD
As NAND process technology advances, the chips themselves become increasingly smaller. As they shrink, the chips also become less reliable at holding data. There are a number of ways to mitigate this problem, including using Error- Correcting Code (ECC). As explained above, any time we give the controller more work to do, it requires more room on its “work bench” to do it efficiently.
The 840 Series represents the first consumer SSD to implement 3-bit/cell MLC (also called TLC) technology. This technology, as its name suggests, stores 3-bits of data per cell, as opposed to the 2-bits per cell that today’s more common Multi-Level Cell (MLC) NAND holds. Because, as just mentioned above, the size of the chips themselves is also shrinking, in effect we are squeezing even more information into a smaller space.
This is nothing a good firmware algorithm can’t handle, however. Samsung’s 3-bit/cell MLC-based SSD 840 Series, equipped with mandatory OP, will still far outlast the useful life of the hardware it powers.
OP, while already popular among SSD enthusiasts, will continue to become an important safeguard for SSD performance and endurance as the NAND industry continues to shrink chips to save on cost, increase efficiency, and expand capacities. Samsung sets a lower value for its mandatory OP on the 840 Series than any other competitor, a feat that is possible because of its top-tier NAND chips and superior maintenance algorithms. As the NAND industry continues to evolve, Samsung will continue to be on the cutting edge, and optimizing features like OP is just one of the many ways it will continue to provide SSD users with the best storage devices on the market.