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SLC vs MLC


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(cross posting from SSDPeformanceBlog.com )
All modern solid state drives use NAND memory based on SLC (single level cell) or MLC (multi level cell) technologies.
Not going into physical details – SLC basically stores 1 bit of information, while MLC can do more. Most popular option for MLC is 2 bit, and there is movement into 3 bit direction.

This fact gives us next characteristics:

  • SLC provides less capacity
  • SLC is more expensive
  • SLC is know to have better quality cheap

Along with that there is also limitation on amount of write operations. SLC can handle about 100,000 write cycles, while MLC is 10,000 ( the numbers are rough, and changing with technology improvement)

No wonder that vendor very quickly come with next separation:

  • SLC for enterprise market ( servers )
  • MLC for consumer market ( desktops, workstations, laptops)

As obvious example here is Intel SSD cards: X25-E ( SLC) is sold as enterprise level card, and X25-M ( MLC ) is sold for mass market. As another example of difference in capacity and price:

  • FusionIO 160GB SLC card price $6,308.99
  • FusionIO 320GB MLC card price $6,729.99

That is for the same price MLC card comes with doubled capacity.

However with increasing capacity difference between MLC and SLC is getting fuzzier. For MLC most critical part is software (firmware) algorithm which ensures a uniform usage of available NAND chips, and with bigger capacity it is much easier to implement.
This problem with handling lifetime and manage write cycle for MLC opened way for hardware solution like SandForce controller and recently Anabit announced “Memory Signal Processing (MSPâ„¢) technology enables MLC-based solutions at SLC-grade reliability and performance”.

Also important is increasing capacity for MLC devices, for example, if we take 10,000 writes vs 100,000 writes than to provide the same life time MLC would need about 10x more capacity, and
it seems not problem. I expect soon we will see MLC cards with 1600GB, which ideally will have the same lifetime as SLC 160GB cards.

On this way interesting to see Intel announces enterprise line for SSD card will be based on
eMLC
( enterprise MLC ), where each cell has 30,000 writes lifetime and with maximal capacity 400GB

So it seems market is gradually moving into “MLC is ready for enterprise” direction, and sounds as good option to have devices with high capacity and reasonable price in near future.

Some articles on this topics:

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Vadim Tkachenko

Vadim leads Percona's development group, which produces the Percona Server, Percona Server for MongoDB, Percona XtraDB Cluster and Percona XtraBackup. He is an expert in solid-state storage, and has helped many hardware and software providers succeed in the MySQL market.



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