Flash memory is based on binary, but the capacity of SSDs sold in the market has options other than binary. Take the 200GB SSD as an example, there are 200GB, 240GB, 250GB, and 256GB products. The flash memory used by these SSDs is mostly binary. Then, in addition to 256GB, other products with insufficient capacity (200/240/250 GB), why come, is the more capacity, the more cost-effective? Let us uncover this veil from different angles.
The smallest read/write unit for flash memory is PAGE, which has 2KB, 4KB, 8KB, and the current mainstream 16KB. Because the storage voltage of the flash memory, the elapsed time will be slowly lost, and it will be damaged after a long time. The nature of the flash memory will cause errors when data is read. Therefore, the SSD has data. Wrong demand. The master uses ECC to verify error correction, ensuring read and write performance and data security, and extending the life of the hard disk. For this reason, the flash memory provides extra space in addition to the user capacity of each PAGE, such as the Micron 64-layer 3D TLC configuration (16KB+2208B) PAGE, and the extra 2208B, which can be used to store the ECC check code. The larger the space, the lower the Code Rate that ECC can support, and the better the debugging capability, but the flash chip area will increase and the cost will increase. Of course, users can't use this extra storage space.
The unit above the PAGE is the block BLOCK, which is the smallest erase ERASE unit. A block consists of hundreds or thousands of PAGEs. Flash memory generates a bad block BAD BLOCK during production and use. There are two types of bad blocks, one of which is present at the factory. When the factory test, too many bad blocks of particles will be regarded as defective. The other is that as the usage time increases, the flash memory will wear out WEAR problems, causing bad blocks to appear gradually, and the available flash memory space will gradually decrease. So when the flash is designed, it will add more blocks (for example, 10%) than the claimed capacity to cope with the bad blocks and the OP.
The firmware of the SSD requires additional space to store system data and perform garbage collection GC. These extra reserved spaces are called OP (Over Provisioning). The larger the OP, the better the random speed. The lower the Write Amplification, the longer the SSD life, but it takes up more flash space. When performing garbage collection, the size of the OP is like the vacancy of playing a digital push game. The more vacancies, the fewer steps you need to move, and the faster and more efficient the solution. The relationship between the size of the OP and the efficiency of garbage collection is similar. Usually enterprise-class or high-end SSDs require large OPs to meet speed and service life requirements at a higher cost.
Let us clarify the difference between GB (Gigabyte) and GiB (Gibibyte). The storage products on the market, such as the capacity of solid state drives, are in GB or TB, while flash memory or memory is in GiB or TiB. One is decimal and the other is binary:
As can be seen from the above table, 1GiB actually has 7.37% more capacity than 1GB. This extra capacity provides basic OP space for the firmware. For example, a full-size solid-state hard drive product, the firmware actually still has at least 7.37% OP available.
As for the capacity standard, the Solid State Technology Association JEDEC clearly defines the relationship between SSD capacity and the number of logical blocks LB in its JESD218A standard. Take 4KB logical block size as an example.
4KB logical block number = 2646 + (244188 x user usable capacity GB)
For example, a 256GB SSD, user addressable to 2646 + (244188 x 256) = 62514774 4KB logic blocks
In the market for SSD products, in addition to the OP, the other is the use of flash granules with more bad blocks at the factory, so-called black and white, rather than the original film flash granules. Such flash memory, if paired with appropriate firmware, can be applied to the cost-oriented low-end SSD market.
The firmware can take advantage of the dynamic remaining space and use it as an OP. If the user is pursuing performance and longevity, not capacity, you can set the total user capacity to be less than the product capacity when you are doing a partition (Drive Partition). This will ensure that you leave space outside the partition and give more firmware. Used as an OP to increase hard drive performance and service life. Product capacity is a market specification and cannot be changed by the gradual increase of bad blocks. Bad blocks that increase over time will only gradually reduce the available OP space. Usually the firmware will detect that once the available OP space is below a certain value, the SSD can no longer be used.
In summary, SSD capacity depends on the flash granules used, the size of the OP, and the positioning of the product. High-end products use larger OPs to meet performance needs. Therefore, the capacity of the SSD is not necessarily as high as possible. When the user purchases, he can think more about comparison and spend the money on the demand.
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