Less is more: The rest of the 840 EVO
Thanks to its 128Gb TLC NAND, our 840 EVO 500GB sample has the highest physical storage density we've seen in an SSD. Cracking open the 7-mm, 2.5" case reveals a tiny circuit board populated with just four flash packages.
Each of those packages has eight individual NAND dies lurking inside. This incredible density allows the terabyte model to be built with just eight packages. It looks like that model uses the same diminutive circuit board, too. Flipping the PCB reveals solder points for more flash.
This side of the board houses the EVO's updated MEX controller. Like its MDX predecessor, the chip has three ARM-based cores. The clock speed of those cores has been boosted from 300 to 400MHz, making the MEX controller more powerful than what's found in the high-end 840 Pro. Samsung has also increased the amount of "hardware automation" by processing more commands directly on the controller rather than in the firmware. We've asked Samsung if this automation involves custom logic or simply makes better use of the existing ARM cores, but we don't have an answer yet.
Samsung has tweaked the 840 EVO with an eye toward providing better performance at the low queue depths typical of consumer workloads. New flash management algorithms have been added to the firmware, and they employ more advanced signal processing techniques to eke every last bit of life out of the NAND. Signal processing and error correction are essential to maintaining endurance as flash process geometries shrink, according to Samsung. The firm appears uninterested in using SandForce-style write compression to reduce NAND wear.
To prevent overheating in cramped notebook chassis, the 840 EVO includes a Dynamic Thermal Guard feature that throttles performance when temperatures exceed 50°C. Samsung hasn't explained exactly how this built-in protection works, but the scheme likely involves scaling back the clock speed of the controller's ARM cores.
256-bit AES encryption support is also built into the MEX controller. Right now, the 840 EVO supports full-disk encryption gated by an old-school ATA password. Support for encryption via the Windows 8 eDrive and TCG Opal specifications is planned for a firmware update due in September.
The only other controller elements worthy of note are those that allow the EVO to double the maximum storage capacity of the other members of the 840 family. In addition to adding support for denser 128Gb flash dies, Samsung tuned the controller to work with the extra DRAM cache required to manage a terabyte of flash. Our 500GB drive has 512MB of LPDDR2 memory onboard, and the 1TB model sports an even gig of RAM.
|Max sequential (MB/s)||Max 4KB random (IOps)||Price||$/GB|
|120GB||3GB||8 x 128Gb||540||410/140||94,000||35,000||$110||$0.91|
|250GB||3GB||16 x 128Gb||540||520/270||97,000||66,000||$190||$0.76|
|500GB||6GB||32 x 128Gb||540||520/420||98,000||90,000||$370||$0.74|
|750GB||9GB||48 x 128Gb||540||520/420||98,000||90,000||$530||$0.71|
|1TB||12GB||64 x 128Gb||540||520/420||98,000||90,000||$650||$0.63|
The table above outlines the specifications for all the members of the 840 EVO family. I've estimated the die configurations and am awaiting clarification from Samsung, but the math works out.
With eight parallel NAND channels and the ability to address four chips per channel, the EVO's controller needs at least 32 dies for optimal performance. At 128Gb (16GB) per die, only the 500GB and larger models deliver on the drive's full potential. The 120 and 250GB models have lower write performance ratings.
Note that the performance table includes two sequential write speed figures for each model. The first refers to the speed of the TurboWrite SLC cache, while the second applies to data written directly to the TLC main storage. The 120 and 250GB variants have the most to gain from TurboWrite, but even the larger capacities should enjoy a healthy performance boost. Unfortunately, Samsung doesn't specify whether the EVO's random write performance ratings apply to the drive's SLC or TLC zones.
Based on specifications and pricing, the 120GB model looks like a pretty raw deal. The cost per gigabyte drops substantially if you pony up for the 250GB model, and the terabyte derivative looks like a steal, relatively speaking. It wasn't so long ago that $600 SSDs offered only 80GB of storage.
Given the 840 EVO's affordable price tag, it's no surprise that the drive is covered by a three-year warranty. Five-year coverage is typically restricted to high-end models like the 840 Pro.
The warranty length doesn't tell you how many writes the drive can endure before the flash burns out. Samsung has been reticent to publish total-bytes-written or gigabytes-per-day specifications for its SSDs, and that trend continues with the 840 EVO. However, if the firm's 2,500-cycle cycle estimate for the NAND is accurate, the drive should be able to endure hundreds of terabytes of writes—plenty of endurance for consumer workloads.
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