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Samsung 840 EVO: New Platform

Judging by its name, the new drive belongs to Samsung’s good old 840 series but the EVO suffix implies some improvements over the Samsung 840, the previous junior model. In this case, the marketing department has not given due credit to the developers, though. The Samsung 840 EVO is absolutely new. It has a new controller, new memory and new operating algorithms. In other words, we have a new generation of SSDs from Samsung hidden behind an old facade. Samsung doesn't market the new SSD as an 850 series for some reason although it would be quite accurate given the internal design. Let's take a closer look at it, by the way.

The Samsung 840 EVO is based on Samsung's new MEX controller. The basic architecture of the chip has remained intact. Like the MDX controller in the earlier 840 series SSDs, it incorporates three ARM Cortex-R4 cores. The clock rate has grown by a third to 400 MHz, which improves the rate at which random-address operations are processed. The MEX controller supports SATA 3.1, so the Samsung 840 EVO can improve performance under heavy loads by queuing TRIM commands.

One more advantage of the new controller is its improved support for 256-bit AES encryption. Samsung SSDs could encrypt data earlier, but the 840 EVO introduces compatibility with the Trusted Computing Group Opal and Microsoft eDrive specifications. Thus, users of this drive can enjoy simple hardware encryption which can be managed from the OS, e.g. via the standard Windows tool BitLocker. Earlier, such encryption used to be enabled via an ATA password in the mainboard’s BIOS. That was not always possible or convenient.

The second fundamental innovation in the 840 EVO design is the new type of TLC flash memory. The drive's predecessor Samsung 840 was the first SSD with TLC NAND flash that could store 3 bits of data in each memory cell. Samsung seems to have found this kind of memory appropriate for SSDs although some people argue that it's not very reliable, being rated for only 1000 rewrite cycles. TLC flash memory did well in the Samsung 840 drive which proved to be quite reliable in practice. Many experiments conducted by enthusiasts have shown that you can write 250 (and even more) terabytes of data even with the 120GB version of the drive. It means that Samsung's TLC-based SSDs are going to last for several years in an average desktop PC.

Interestingly, other manufacturers have not yet released any products with TLC NAND, so Samsung seems to have some proprietary technologies that help use 3-bit flash memory in the most efficient way. And now these technologies are improved further as the Samsung 840 EVO features a new type of TLC memory. As opposed to the 840 model's 21nm flash memory, it features 19nm memory with Toggle Mode 2.0 DDR interface that has a peak bandwidth of 400 Mbps.

TLC NAND flash memory can store 50% more data than regular MLC NAND flash in a semiconductor device of the same complexity. Thus, it helps reduce the manufacturing cost of SSDs and lets the manufacturer sell them cheaper at the same profit. Having solved any reliability issues related to TLC NAND flash, Samsung can now keep on improving the manufacturing technology and make its SSDs even more profitable.

With the transition to the new manufacturing process Samsung has switched from 64- to 128-gigabit semiconductor dies. Micron's MLC NAND flash with 128-gigabit dies was also employed in the recently tested Crucial M500. It helps increase the maximum capacity of SSDs to 1 terabyte, so the Samsung 840 EVO uses 128-gigabit dies for the same purpose. However, Samsung's 19nm TLC flash differs from Micron's MLC flash in being structured into 8KB rather than 16KB pages. Samsung's flash memory is erased in 256-page blocks as is typical of 64-bit memory. All of this means that the transition to larger-capacity dies won't increase write and erase latencies much. That's good news since TLC NAND is overall slower than MLC NAND.

And now we should mention one more special feature of the Samsung 840 EVO which is meant to boost its performance. It is called TurboWrite. TLC memory is known to be slower than MLC. The “thinner” tech process lowers performance even more. As a result, the 19nm memory installed in the Samsung 840 EVO turned out to be too slow to make a fast SSD as Samsung wanted. The solution found by Samsung engineers is in using a cache which emulates SLC NAND flash memory. It stores 1 bit of data in each memory cell but works with extremely low latencies. The idea of such caching is quite obvious. Indeed, it has been long used in products from Toshiba and SanDisk. Samsung’s implementation differs in that the cache is only employed for writing. The high-speed SLC mode is limited to a part of TLC memory in the reserve pool inaccessible by the user. Even a small cache with SLC simulation turns out to be enough for a considerable boost in performance.

Depending on the drive’s capacity, the TurboWrite cache varies from 3 to 12 gigabytes.

When data is written to the drive, it is first written into the SLC cache and later, during an idle period, transferred into the TLC area. It means that the Samsung 840 EVO is going to deliver an extremely high writing performance at first but, if the amount of data exceeds the TurboWrite cache size, the speed goes down since the drive has to save data into slow TLC instead of fast SLC memory. Writing large amounts of data continuously isn’t a typical load for everyday applications, though. That’s why we can expect the TurboWrite technology to be most beneficial in practice.

The TurboWrite cache makes up for the reduced reserve pool of memory cells. The caching of write operations allows to consolidate them and reduce the write amplification level. So, even though the reserve pool is only 5.6% of its total storage capacity, the Samsung 840 EVO should be as reliable as other consumer-class SSDs.

Summing up our overview of the new SSD platform implemented in the Samsung 840 EVO, let’s take a look at the specified speed compared to Samsung’s previous products:

As you can see, the new SSD is very close to the Samsung 840 Pro in its specs. Its good write speed is ensured by the TurboWrite technology whereas its good read speed relies on the new Samsung MEX controller. Well, it wouldn’t be exactly correct to view the Samsung 840 EVO as the 840 Pro model selling at the price of the Samsung 840. There are a lot of nuances the manufacturer doesn’t talk much about. It is to reveal them that we are going to test a 250GB version of the Samsung 840 EVO drive. Let’s have a look at its design first.

 
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