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最終更新日 May 17, 2023

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What Are Cell Levels?

Explains How NAND Cells Can Be Used By SSDs And What Those Impacts May Be

Summary

NAND, which is the storage media in most SSDs, stores data in cells. Each cell holds a charge that represents data. We’ll relate a cell to a battery as each cell layer is explained.

SLC

  • SLC means Single Level Cell and is the basic use of a NAND cell.
  • SLCs can store one bit of data per cell:
    • 0 or 1
  • This requires two states to measure:
    • One state can be empty
    • The other state is NOT empty.
  • Charging the battery (or writing the cell) is easy
    • Anything greater than empty changes the bit from 1 to 0.
  • Checking the charge (or reading the cell) is also easy, only one check:
    • Checking if cell is empty,
    • If true, bit is 1
    • If false. bit is 0

SLC is the fastest and most costly NAND (dollar per GB).

MLC

  • MLC means Multi-Level Cell and is poorly named.
  • MLCs can store two bits of data per cell:
    • 00, 01,
    • 10, 11
  • MLC requires four states to measure:
    • Empty+ Three charge states.
  • Reading and writing is more complex than SLC:
    • Charging (writing) must be to a precise charge, otherwise the wrong bit sequence is saved.
    • Checking (reading) will take more than one attempt.

MLC 'costs less' than SLC, because the added bit doubled the storage with no change to the NAND.
The actual cost is in performance, due to extra precision and checks required, the performance is lower than SLC.

TLC

  • TLC stands for Triple Level Cell
  • TLC can store three bits per cell:
    • 000, 001, 010, 011,
    • 100, 101, 110, 111
  • TCL needs eight states to measure:
    • Empty + seven charge states
  • The charge states are smaller and closer together.
  • Read and writing is more difficult than MLC:
    • Writing required higher accuracy
    • Reading requires more checks to get the correct range (data).

TLC costs less then MLC, because the added bit gives 33% more storage per cell.
The cost is, again, in performance, due to more precision and checks.

(See a trend?)

QLC

  • QLC stands for Quad Level Cell
  • QLC can store four bits per cell:
    • 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111
    • 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111
  • QCL requires sixteen states to measure:
    • Empty + fifteen charge states.
  • States and the space between them shrink even more.
    • Reading and writing get even more difficult from TLC
    • Higher accuracy for writes, more checks for reads.

QLC costs less then TLC, because the added bit fives 25% more storage per cell.
The cost is even more performance, due to even more precision and checks.

PLC

  • PLC stands for Penta Level Cell
  • PLC can store five bits per cell:
    • 00000, 00001, 00010, 00011, 00100, 00101, 00110, 00111, 01000, 01001, 01010, 01011, 01100, 01101, 01110, 01111
    • 10000, 10001, 10010, 10011, 10100, 10101, 10110, 10111, 11000, 11001, 11010, 11011, 11100, 11101, 11110, 11111
  • QCL requires sixteen states to measure:
    • Empty + thrity-on3 charge states.
    • States and the space between them shrink so much, our image cannot show the bit sequences in each state.
  • Reading and writing get even more difficult from QLC
    • Highest accuracy for writes and checks for reads.

PLC costs less then QLC, because an extra bit added 20% more storage per cell.
The cost is even more performance, due to even more precision and checks.

Conclusion

Pros

  • As bits are added, per cell, the capacity increases, while keeping the cost of the NAND the same.
    • Companies use the same amount of physical NAND, but you get a higher capacity.
  • More levels give a lower price per GB.

Cons

  • The performance and endurance decrease as bits are added.
    • SSD design and firmware are used to gain the performance loss back.
  • If you are looking for performance, less bits per cell is best.
  • If you are looking for a lowest cost, the higher bits per cell is best.

Questions?
Contact Solidigm™ Customer Support
.

What Are Cell Levels?