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Samsung's 840 EVO solid-state drive reviewed

TLC NAND with a shot of SLC cache
— 7:00 AM on July 25, 2013

Samsung is one of the biggest names in technology. The Korean giant makes everything from vivid 4K televisions to powerful smartphones to cutting-edge washing machines—with accompanying Android apps, of course. You can't throw a rock in an electronics store without hitting at least half a dozen Samsung products.

Your odds of striking a PC equipped with a Samsung SSD are pretty good, too. The company claims 60% of SSD-equipped PCs shipped this year will have Samsung drives. Samsung's SSDs are also popular components on their own. In the US, the firm says it makes up 36% of the market for branded solid-state drives.

A lot of Samsung's recent sales have come from the 840 Series. Over 2.5 million of these entry-level drives have sold worldwide, and for good reason. The 840 Series offers solid performance at extremely affordable prices. It does have a weakness, though. The drive's TLC NAND costs less per gigabyte than the MLC flash typically used in consumer SSDs, but its write performance is somewhat compromised by the higher bit density.

Samsung has attempted to address that shortcoming with its next-generation successor to the 840 Series. The 840 EVO features a faster controller, newer TLC NAND, and a dedicated write cache populated with speedier SLC flash. There's a second layer of caching that leans on system memory, too.

Despite these enhancements, the 840 EVO retains the budget-friendly pricing of its predecessor. We've been testing one pretty much around the clock for the past few days. Let's see how it stacks up.

The finest dual-mode flash
If you look at Samsung's semiconductor businesses, it's no wonder the company is a formidable force in the SSD market. Samsung is the biggest flash producer in the world, after all. It also has expertise in customizing multi-core processors and fabricating DRAM, allowing the firm to produce all the component parts of a modern SSD entirely within its own facilities.

Most of the Samsung silicon inside the 840 EVO is second-generation TLC NAND. These chips are fabbed on a 19-nm process, just a small step down from the 21-nm node used for the 840 Series' flash. The new NAND conforms to the same 400Mbps Toggle DDR 2.0 specification as the old chips.

Perhaps to avoid the negative connotations associated with TLC memory, Samsung refers to the 840 EVO's NAND as three-bit MLC. That's technically correct: TLC flash has three bits per cell, one more than MLC memory and two more than the single-bit SLC stuff. The higher storage density makes TLC NAND cheaper to produce, since more gigabytes can be squeezed out of each wafer. However, it also reduces the NAND's write performance and endurance.

I explained the challenges associated with TLC NAND in our 840 Series review, so I'll just give you the Coles Notes here. Understanding the problem requires some knowledge of how flash memory works. NAND cells are written by causing electrons to tunnel into an insulated floating gate. The negative charge associated with these electrons defines the data in the cell, and the range of possible voltages is independent of the number of bits.

Because there is some cell-to-cell variance in the silicon, writing to the NAND involves a verification step that reads the contents of the cell. Reading entails applying a series of control voltages to hone in on the charge in the floating gate. In TLC NAND, that charge can represent one of eight possible values between 000 and 111. MLC NAND only has to contend with four values between 00 and 01, while SLC is limited to two: 0 and 1. The more bits per cell, the more iterative steps are required to verify the data, slowing the write process.

As flash memory accumulates write/erase cycles, electrons get trapped in the gate insulator, reducing the range of voltages that can be used to represent data. This shrinkage is especially troublesome for TLC NAND, which has to contend with more bits within that narrowing window. Increasing the number of bits per cell lowers its write/erase tolerance.

NAND cells are made even more vulnerable by smaller process geometries, which reduce the thickness of the insulator layer. Samsung claims the 19-nm flash used in the 840 EVO is just as resilient as the 21-nm chips employed by the 840 Series, though. According to the official reviewer's guide, the 840 EVO's NAND should endure "at least 2,500" write/erase cycles. That's an impressive figure for 19-nm TLC chips, but we shouldn't be surprised. Samsung makes a lot of flash, and only the best dies are selected for use in its solid-state drives. The firm says its SSD-grade TLC NAND has 20X fewer bad blocks than the flash bound for lesser devices, such as USB thumb drives.

Although most of the 840 EVO's flash is configured with three bits per cell, a small slice of each die is addressed as single-bit SLC NAND. This portion of the flash is dedicated to TurboWrite, a buffering technology that caches host writes before transferring them to the drive's main storage. The SLC cache promises higher write speeds than the TLC main storage, particularly for sequential transfers.

TurboWrite funnels all downstream traffic through the SLC cache. Writes only bypass TurboWrite if the buffer is full. When the drive is idle, cached writes are transferred out of the buffer and into the TLC NAND.

The size of the TurboWrite cache varies based on the 840 EVO's total capacity. The 120 and 250GB models have 3GB reserved for TurboWrite, while each higher-capacity variant adds another 3GB to the cache. According to Samsung, 3GB is enough to accelerate "everyday performance scenarios." Even the smallest TurboWrite cache is considerably larger than the 1GB buffer employed by SanDisk's Extreme II SSD. The Extreme II is the only other SSD we've seen with an SLC cache.

Due to the lower bit density of SLC NAND, the flash footprint of the TurboWrite cache is three times higher than the buffer's capacity. For example, the 250GB model's 3GB SLC cache consumes 9GB of TLC NAND. With the exception of the 120GB model, the TurboWrite cache monopolizes 3.5% of the drive's total flash capacity. The smallest member of the family devotes 7% of its flash to TurboWrite.

TurboWrite is responsible for the 840 EVO's slightly lower capacities (250GB instead of 256GB, for example). The 840 Series comes in similar sizes, but it doesn't have a fancy SLC cache. The older drive does, however, overprovision additional capacity to replace bad blocks caused by cell failures. Samsung says its initial TLC endurance estimates were conservative, and that the NAND is robust enough to repurpose the extra overprovisioned capacity for TurboWrite.

With just about all incoming data running through the SLC cache, that portion of the flash is going to chew through a lot more write/erase cycles than the TLC main storage. Single-bit NAND has much higher write endurance than its three-bit counterpart, though. Consolidating smaller cache writes into larger blocks before moving them to main storage could also make more efficient use of the TLC NAND, prolonging its life. We're waiting to hear back from Samsung about whether TurboWrite repackages writes before they're passed to the TLC NAND—and whether the SLC cache is moved around on the die to spread the wear evenly.