Review: Samsung 845DC EVO 480GB

Introduction and specificationThe Samsung 845DC EVO is an enterprise-class SSD optimised for read-centric workloads, common in database and server applications. As with the 840 EVO, and the PM843, an OEM-only model, it uses TLC flash memory, or 3-bit MLC NAND as Samsung prefers to label it.TLC flash memory is cheaper to produce than the 2-bit MLC NAND currently used in most consumer drives, but it has some downsides, namely lower endurance and performance.As the world's largest NAND producer, by using its sheer manufacturing weight, Samsung has managed to mitigate most of these issues with some tweaks that boost the drive's longevity and stability.A closer lookThe 840 EVO series encompassed five different storage capacities, from 128GB to 1TB. There's only three with the 845DC EVO though: 240GB, 480GB and 960GB. Four 128GB chips make up the total capacity of the 480GB model we tested – which retails at around the £340 mark (around $530, AU$630), or a touch under – with 32GB reserved for over provisioning. There's 512MB of LPDDR2 cache, with 1GB in the 960GB model.As before, Samsung is using Toggle 3-bit MLC NAND, with a 3-core MEX controller. As with the Saumsung 850 Pro, the controller has been bumped up to 400MHz.The warranty is now five years, rather than three with the 840 EVO, which was low for an SSD aimed at enterprise use. It doesn't match the ten years Samsung offers with its 850 Pro (which was offered as a last-minute decision, made after rival SanDisk announced the same warranty period on its Extreme Pro SSD).SpecificationPerformance is quoted as up to 530MB/sec read speeds for all three models, with 410MB/sec write speeds for the 480GB and 960GB models, and 270MB/sec for the 240GB SSD.Random 4K QD32 read IOPS are quoted as 87,000 for all three models, and for 4K random writes, a much lower 14,000 for the 480GB and 960GB models, and just 12,000 IOPS for the 240GB model.A small portion of the 840 EVO storage was reserved for something called TurboWrite, where a portion of the flash memory was treated as SLC flash, with each cell only written to once, and used as a cache, reducing the overall number of writes to the rest of the TLC memory. However, this is not present with the 845DC EVO. An important addition is power-loss protection, implemented by an extra set of capacitors inside the drive. If power is pulled from the 845DC EVO for any reason, this feature gives it just enough time to flush data from the cache to the storage. This was not present in the 840 EVO.As an enterprise drive, the big focus is on read performance. Samsung envisages the 845DC EVO being used in servers, where more data will be read than written – a web server or database, for example. More often than not, the files are likely to be small, not massive chunks, so sequential performance is less relevant than IOPS and random 4K read speeds.TLC flashAll flash memory can only be written to a number of times, quantified as program-erase (P/E) cycles, whether it's in a USB stick, SSD, or storage in a phone or tablet.This is unavoidable. In order to erase a cell, a voltage needs to be applied to it. Each time this is done, a cell becomes ever so slightly less sensitive to the range of voltages it can accommodate (which represent binary values). Eventually it can no longer operate.Early SSDs earned a reputation for being notoriously expensive, mainly because of their use of extremely high-end SLC (single-layer cell) flash memory, which can endure the highest amount of writes.With SLC flash memory, each cell can only be in one of two states, on or off, binary 0 or 1. It only needs to represent two voltage values, and when a bit is changed, a single cell is written to once. For 2-bit MLC, there are four possible binary states: 00, 01, 10 and 11. Therefore there are four voltage values it has to represent. The cell will undergo twice as many writes to it.It follows that three-bit flash memory has eight binary states, ranging from 000 to 111. Erasing data means making more writes to each cell, so faster degradation, while at the same time, the cells need to be sensitive to a wider range of values. The result is exponentially fewer P/E cycles per cell.Of course, for a given capacity, you need a lot of flash memory silicon for an SLC drive, which drives prices right up. Much less is required in a TLC drive though.A cell in an SLC drive might be good for 1,000,000 P/E cycles, but this drops to only 1,000 for an average TLC flash chip. With the 845DC EVO, Samsung has managed to triple this, to around 3,000 P/E cycles per cell.Longevity and performanceAs the Samsung 845DC EVO is an enterprise drive, it has been designed with maximum lifespan in mind, despite being built with TLC NAND. SSD reliability can be measured in a number of ways, but total bytes written (TBW) is now the standard figure provided by most manufacturers. With most SSDs this doubles with each capacity increase.The 240GB 845DC EVO model is rated for a respectable 150TB of writes, the 480GB lasts for a whopping 300TB, while the 960GB model lasts for a truly staggering 600TB.That's a lot of use. Taking the 480GB model as an example, you get 60TB of writes for each year of the five-year warranty, which equates to 164.38GB of writes, every single day. With the 845DC EVO installed in your PC as an operating system disk at home, you'd find it very hard to reach that limit with "normal" daily usage. Those limits will also be hard to reach in the majority of scenarios where the 845DC EVO is used in a server. Most servers are about sending data to clients rather than writing data constantly. Even the types of usage from a heavily loaded file server or database might prove the exception rather than the rule, when dealing with a single SSD.PerformanceSamsung sent us a pair of 480GB 845DC EVO drives, so we tested them in both single and RAID 0 configurations, using all the usual software.CrystalDiskMark results matched Samsung's claims for sequential reads, hitting 543MB/sec. Writes levelled off at 438MB/sec. With the drives in RAID 0, these speeds roughly doubled, as expected, with 1117 MB/sec read and 873MB/sec write speeds.Random 4K QD32 read speeds hit 82,048 IOPS, slightly lower than Samsung's claim, but within a wide margin of error. With the two drives in RAID 0, this went up to 144,640. QD32 write speeds were 320 MB/sec, in excess of Samsung's claims.AS SSD is a tougher test, since it's a tool which uses mostly incompressible data. In this benchmark the results mostly tallied with the CrystalDiskMark scores, with 522MB/sec sequential read speeds and 437MB/sec write. Read access times were 0.043ms reading and 0.035ms writing.The scores place the 845DC EVO in the top segment for reading, but roughly around the middle of SSD performance charts for writing. Other firms' drives will outperform it on both fronts, but it's still a fast drive.PricingOne final note on the pricing – it's disappointingly high, with the 480GB version pitched at about £340 (around $530, AU$630), as we mentioned at the beginning of this review.TLC flash is supposed to be cheaper to produce, but it seems these savings are not being passed on to the consumer. This is typical of many enterprise IT products, which are less price sensitive than off-the-shelf consumer drives.VerdictWe likedThe 845DC EVO's sequential read speeds are excellent, and right up there with the best SSDs on the market. Random read IOPS are great as well.The endurance is excellent too. Samsung is right on the money by pushing hard to improve this area of SSD manufacturing. The sheer volume of data that can be written to the 845DC EVO per day is staggering.We dislikedThe prices seem slightly out of proportion with the technology. The 840 EVO was well-priced, so it seems the 845DC EVO is moving in the wrong direction.The write speed is not bad, but it's not the best around. These two factors lead me to conclude the 845DC EVO is best for its stated purpose – server deployment with read-heavy workloads – rather than being a suitable SSD for a home PC.Final verdictAs an SSD for home use, the 845DC EVO is not great overall value for money, even compared with Samsung's other faster drives, such as the 850 Pro, but especially compared with drives like Crucial's MX100. The 512GB model of the latter costs about the same as the 240GB version of the 845DC EVO (the larger Crucial drive is even a bit less at some online retailers).This drive does have an extraordinary level of longevity in its favour, though. Samsung is truly making the most of its advantages in NAND production, taking TLC endurance to new heights with the 845DC EVO. The technology is moving forward at breakneck speed, and the Samsung 845DC EVO should silence any criticism of endurance issues with TLC NAND.This longevity, along with the high sequential read speeds, could make the 845DC EVO a good choice as an SSD for server use, depending on the workload.

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