New Flash Technology - Page 2


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To help with the low endurance, TLC manufacturers have increased over-provisioning (the chips are cheaper after all) and improved the controller's wear-leveling. They have also added more error correcting capability. The result is a much more even wear on the chips as well as a pool of "spare" cells that can be used if needed (reallocation). There is an article from 2012 that tested the endurance of a TLC drive, the Samsung 840. For client workloads TLC reduced the total estimated lifespan by about 2/3 relative to MLC, but for the smaller drive, 128GiB, the expected life was still almost 12 years. Enterprise workloads are sometimes more write intensive than client workloads, reducing the lifespan. But don't be surprised if the combination of over-provisioning and wear leveling keep TLC chips competitive.

Recently there was a long-term test of SSDs to see how much data could be written to the drives before they died. They tested consumer drives all with a capacity of around 250 GB (plus or minus). The goal of the test was to determine how much data can actually be written to each of the drives before they die. The test should stress both the flash chips and the controllers. One of the drives was the 240GB Samsung 840 which uses TLC flash chips. They also tested the 250GB Samsung 840 Pro which uses MLC chips. The other drives used MLC and included two drives with a SandForce controller. One of the SandForce drives used incompressible data, negating the compressing feature of the controller. The other drive used 46 percent incompressible data allowing the controller to do some compression. This allows the drive to perhaps write less data than the drive with incompressible data.

The first two casualties of the testing were actually MLC based SSDs. The tests showed that around 700TB, the Samsung 840 was using its over-provisioned cells at a steady rate but was still functioning. The 840 started logging reallocated sectors after around 100TB of data written and started steadily reporting them after that point. It also started allocating more spare area to replace bad blocks, as one would expect with a TLC based drive. It was still functioning after 900TB but died shortly thereafter, not quite making one Petabyte. In comparison, the Samsung 840 Pro, which uses MLC, managed to write just a little over 2.4PB of data before failing (one of the last drives standing).

There are a number of TLC drives available for the consumer market. A quick glance at Newegg on 4/5/2015 showed that TLC drives were about $0.50/GB for the range of 250GB to 1TB in capacity. The MLC drives start at just under $0.50/GB at 120TB but decrease to about $0.35/GB at 240TB. For drives up to 1TB the $/GB rises slightly but is still under $0.40/GB.

Even though MLC drives are a touch less expensive than TLC drives, it is predicted that TLC chips will start to dominate the flash market in 2015, as shown in Figure 1 below.

TLC Usage ratio rise

Figure 1: Estimate of flash chip types in the market
[Source: http://www.thessdreview.com/daily-news/latest-buzz/silicon-motion-announces-sm2256-worlds-first-6gbs-ssd-controller-supporting-tlc-nand/ (used with permission)]

By the end of 2015 it is predicted that TLC will make up 50 percent of the flash chip market. But also notice that the market share for SLC chips also picks up to about 10 percent at that time.

TLC is even making its way into the enterprise world with the launch of the Samsung 845DC EVO. When it was released, it was about $1/GB. However, now you can get the 960TB Samsung 845DC EVO drive for about $660 ($0.69/GB).

3D Chips

The original flash chips were planar. That is, the cells were all on the same level within the chip. However, companies realized that they could increase the chip density by going vertical. This led to the development of 3D flash chips. These chips are now available and are helping to increase the density and drive down the $/GB.

As with TLC, Samsung is the most aggressive company in terms of bringing 3D NAND to market. As the chart in Figure 2 illustrates, 2015 will be the year that 3D flash drives will appear from a number of vendors.

3D Nand Timing

Figure 2: Timing of 3D NAND chips and drives
[Source: http://www.theregister.co.uk/2014/12/16/3d_nand_flash_of_the_future/ (used with permission)]

As the chart explains, Samsung is already shipping drives that use 32 layers of MLC cells in the flash chips. These are the Samsung 850 PRO and 850 EVO. The Samsung 850 Pro has a slightly higher price point than the Samsung 850 EVO, as shown below in Table 2 (prices taken from Newegg.com on 4/5/2015).

Table 2: Samsung 3D flash drive comparison

Drive TypeCapacityPrice$/GB
Samsung 850 Pro128GB$96.00$0.75/GB
Samsung 850 Pro256GB$159.00$0.62/GB
Samsung 850 Pro512GB$290.00$0.566/GB
Samsung 850 Pro1TB (1,024GB)$552.00$0.537/GB
Samsung 850 EVO120GB$73.00$0.608/GB
Samsung 850 EVO250B$110.00$0.44/GB
Samsung 850 EVO500GB$210.00$0.42/GB
Smasung 850 EVO1TB (1,024GB)$390.00$0.381/GB

As you can see, 3D MLC NAND is currently priced very close to MLC drives, and they are expected to come down in price.

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