One of the most significant shifts in semiconductor manufacturing in the past five years has been Samsung's rise to power in the NAND flash market. While Samsung has been manufacturing drives for years, the company has only recently begun to focus on the consumer market.
Thus, even in a market where drive prices are falling rapidly, the Samsung 840 250GB stands out. At £135 for a 250GB SSD, the 840 Series is superb value for money by anyone’s standards. Samsung is hitting this lower price point thanks to a new kind of higher-density NAND flash, called TLC (triple-level cell), but does so at some cost to speed.
First, some background on flash prices. There are two ways for a manufacturer to drive flash prices lower. It can move to a smaller process node and manufacture more NAND flash per silicon wafer, or it can increase the amount of information each memory cell is capable of storing. Early SSDs and industrial/enterprise products used single-level cell, or SLC flash. SLC flash is extremely fast and capable of enduring over 100,000 program/erase cycles, but it’s extremely expensive.
The consumer SSD market didn't take off until the first MLC (multi-level cell) NAND became available. MLC drives store two bits of data per cell. MLC flash isn't quite as quick as SLC, and it's typically rated for 3,000 to 5,000 program/erase cycles, but it's far more economical.
Now, there is triple-level cell NAND, which, as the name implies, can store three bits per memory cell. Manufacturers have actually been working on TLC NAND for years, but fabrication and longevity issues delayed the technology's introduction to the consumer SSD market, despite the cost advantages. The problem with TLC NAND, at least initially, is that the number of program/erase cycles is only in the 1,000 to 1,500 range. That's low enough to be potentially problematic, even in consumer products. The drives are also slower than SLC or MLC products – it takes longer to read and program each bit.
Samsung has kept silent on the exact specifications of its TLC NAND, which hasn't exactly helped dispel the concerns over long-term TLC flash reliability. The good news is that months of real-world stress tests by the greater online tech community have all been positive. Multiple individuals and websites have bought the drives and hammered on them for several months using stress tests purposefully designed to push TLC NAND's program/erase cycle count. The results have pointed to excellent longevity, with drives capable of lasting 5+ years in standard client workloads.
Our review unit was tested using an Asus P877V-Deluxe motherboard with 8GB of DDR3-1600 RAM and an Intel Core i7-3770K CPU. The P877-V Deluxe offers multiple SATA controllers from Intel and Marvell; the 840 Series was connected to Intel's 6G SATA port.
The performance figures for AS-SSD and SiSoft Sandra reflect a drive's performance in a particular type of data workload. Sequential read/write tests measure an SSD's capabilities when reading or writing a large block of contiguous data. A single large movie or ISO image will test a drive's sequential performance (assuming that the target drive isn't badly fragmented). In AS-SSD, the 840's sequential read performance matched that of the higher-end OCZ Vector (517MBps vs. 509MBps). The Mushkin Chronos fell roughly ten per cent behind, at 466MBps. The 840 Series fell back sharply in sequential writes; its 244MBps performance put it in third place behind the smaller Mushkin Chronos (266MBps) and the OCZ Vector at 495MBps.
The random read/write performance data from SiSoft Sandra that we also quote is a measure of a drive's sustained performance when reading and writing a contiguous block of information to a randomly chosen location. These metrics are important because they collectively measure the different types of storage tasks an SSD or HDD performs, even if they don't represent user workloads. In this test, the Vector, Mushkin, and Samsung 840 all perform well in terms of reads (530, 507, and 530MBps respectively). Write tests again show a big gap between the three, with the Vector coming in on top at 509MBps, compared to 276MBps for the Mushkin Chronos Deluxe and 246MBps for the Samsung 840.
The 4K read/write tests measure the performance of an SSD or HDD when reading and writing small chunks of data. These small read/writes are vital to the everyday performance of a storage solution. The "64 Threads" test in AS-SSD means that the benchmark program spins off 64 separate 4K read/write tasks. This stretches the controller's ability to manage such workloads, but also provides a more realistic performance metric – an operating system is constantly reading and writing data to multiple services and programs simultaneously.
In our 64 Thread 4K test, the Samsung 840's 330MBps read rate was nearly double that of the Chronos' 181MBps, but still behind the OCZ Vector's 359MBps. The Samsung 840 slipped to third place in 64K writes again, with a score of 186MBps compared to the Mushkin Chronos (214Mbps) and OCZ Vector (304 MBps).
PCMark 7 is a different type of test. The benchmark uses real storage workloads created by recording traces of hard drive activity when playing games, loading music or video, or copying files. These traces are used to measure the performance of storage products in comprehensive real-world scenarios. Here, the Samsung 840 and Mushkin Chronos Deluxe essentially tied at 5,265 and 5,244 respectively. The OCZ Vector remained slightly ahead, with a storage score of 5419. The narrowness of the gap reflects one of the underlying facts of SSD use – while moving from an HDD to SSD is an enormous upgrade, the differences between SSDs for general workloads don't feel very large.
Honestly, I was dubious of Samsung's plans to bring TLC NAND to market when I first heard about the 840. The drive's performance is strong – definitely competitive with the lower-end tier of MLC – and the stress tests that have been done in the months since the 840 Series debuted have illustrated that the drives can handle the rigours of regular use.
Samsung's got a winner here. Lingering concerns over NAND reliability might still push some buyers towards MLC drives, but the TLC NAND inside the 840 has proved its worth. Some of the budget 120GB SSDs we've examined are still great options for users who need to keep prices well below £100, but £135 for a 250GB drive is a great deal, and the Samsung 840 Series easily scoops one of our Best Buy awards.