We will review the different architecture of the SSD drives that are on the market. In a previous article the differences between Hard Disk Drives (HDD) and Solid State Drives (SSD) where polarized, feel free to take a look at that article here.
Let’s first discuss two different architecture differences for SSD namely SLC (single level cell) and MLC (multi-level cell).What distinguishes these two drive types is how many bits they can store in each cell. SLC drives store one bit per cell and respectively MLC drives store 2 bits per cell. MLC and SLC have different characteristics in fields such as performance, reliability, and price when choosing a SSD drive a couple of items should be considered which include costs, data throughput, drive reliability and longevity
Life span or endurance is the first distinction I would like to share with you. Life spans for solid state drives can be referenced by how many times data can be written and erased on the same cell; this is commonly known as cycle counts. SLC drives are rated for 100,000 cycle counts before the cells start to degrade and the disk is likely to fail. MLC drives have a considerably shorter life span and are only rated at 1,000 cycle counts. This is a big factor for larger data processing environments that continually cycle data on their drives. Aside from their limited write/erase cycles SSD drives are still susceptible to other hardware failures just like HDDs.
Next let’s look at the performance differences between these two drivesSLC drives perform at a higher level of functionality. Going through cells of data that only have one bit is simpler as opposed to two or four bits. Resulting in SLC drives being faster than MLC
drives. We can use one study that compares SLC vs. MLC drives to give us a basic understanding of the read write speeds on these SSD’s. The study clocked access times of the SLC drive at 25us and MLC drives at 50us. They also observed write times for the two drives, saying that MLC drives can write 600–700us and SLC drives can go as fast as 200-300us. With this information so far we can see that SLC drives have a notable advantage in speed and reliability.
|Read||25 us||50 us|
|Program||200-300 us||600-900 us|
|Erase||1.5-2 ms||3 ms|
Chart from http://WWW.smxrtos.com/articles/mlcslc.html)
The price between MLC and SLC drives can be very steep. I used two Transcend SSD’s to express the financial contrast of MLC and SLC drives. Transcend Information Inc. is acorporation that has been manufacturing different kinds of memory and Storage products out of Taiwan since 1988. First I looked at their 64GB SATA MLC Internal Solid State Drive, which is listed at $131.60. If we break down that price to gigabyte ratio we can see that it will cost $2.5 per gigabyte. The other drive was a Transcend 16GB 2.5” SATA SLC Industrial Solid State Disk and it is priced at $227.80. For the SLC drive you pay a hefty $14.23 per gigabyte. The conclusion is you will typical pay 10 times as much per gigabyte for SLC drives. So it’s no wonder MLC drives have been getting more attention by developers and consumers.
On a side note, SLC generate less heat by operating at lower voltages. Efficiently is always a pulse for any environment. Regardless, MLC has been the big player in the market. With more advancement being made on MLC drives it seems people feel they have a strong foundation to grow into industry settings.