Samsung 840 White Paper page 20

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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,