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Clash of the terabyte titans: Crucial M500 960GB vs. Samsung 840 EVO 1TB

The best values at the top of the market

I feel a little silly saying this, but the Crucial M500 960GB and Samsung 840 EVO 1TB are two of the best values in the SSD market. Both drives sell for north of $500, which puts them firmly in high-end territory. However, they also have loads of storage. If you do the math, these terabyte flagships actually cost less per gigabyte than lower-capacity models with cheaper price tags. And they have higher performance ratings, too.

Considering the manufacturers, we probably shouldn't be surprised that these drives are such relative bargains. Samsung is the largest producer of flash memory in the world, while Crucial is the consumer brand of memory giant Micron. Both firms have cutting-edge NAND fabrication facilities, and their SSD divisions are first in line for the latest chips. Those divisions no doubt get a nice family discount, too.

The question, of course, is which of these flash giants makes the best SSD? We've pitted the M500 960GB against the 840 EVO 1TB to find out. We've also included smaller variants from each lineup to provide a little extra flavor for folks with tighter budgets. Join us as we crown the king of the terabyte titans.

TLC vs. MLC flash
Since we've already covered lower-capacity versions of the Crucial M500 and Samsung 840 EVO in earlier reviews, we won't rehash all the details here. I suggest reading those initial articles for a detailed look at each drive's underlying architecture. Today, we'll highlight a few key differences and similarities before moving on to the results of our exhaustive performance testing.

By far the biggest difference between the M500 and the 840 EVO is the flash memory under the hood. Like most consumer-grade SSDs, the M500 features MLC NAND with two bits per cell. The 840 EVO is based on TLC flash that squeezes an extra bit into every memory cell.

Flash cells represent data with a negative charge created by trapped electrons. Because there's some unavoidable variation in the nanoscale characteristics of each cell, all data is verified after it's written. The verification process takes longer with TLC NAND, which must differentiate between eight possible values between 000 and 111. MLC NAND only needs to worry about four values between 00 to 11.

Due to the more involved verification process, TLC NAND typically has slower write speeds than its MLC counterpart. However, the 840 EVO isn't a typical TLC implementation. Although much of its flash has three bits per cell, a portion is addressed as server-style SLC NAND with only one bit per cell. This SLC zone totals 12GB and is reserved for TurboWrite, which caches incoming data to mitigate the slower write speeds associated with TLC flash. Cached data is held in the TurboWrite buffer until the EVO slips into an idle state, after which the data is passed along to main storage.

Model Max sequential (MB/s) Max 4KB random (IOps) Price $/GB
Read Write Read Write
Crucial M500 240GB 500 250 72,000 60,000 $145 $0.60
Crucial M500 480GB 500 400 80,000 80,000 $350 $0.73
Crucial M500 960GB 500 400 80,000 80,000 $530 $0.55
Samsung 840 EVO 250GB 540 520/270 97,000 66,000 $165 $0.66
Samsung 840 EVO 500GB 540 520/420 98,000 90,000 $320 $0.64
Samsung 840 EVO 1TB 540 520/420 98,000 90,000 $570 $0.56

Thanks to TurboWrite, the 840 EVO 1TB has a peak sequential write speed of 520MB/s. Writes proceed directly to the TLC flash if the TurboWrite buffer is full, but that only drops the EVO's sequential speed rating to 420MB/s—20MB/s higher than the M500's peak write rate. At least according to the manufacturers' specifications, the 840 EVO is faster than the M500 not only in every category, but also across the multiple capacities we've tested. In a moment, we'll see if that dynamic holds up in the wide variety of benchmarks that make up our storage test suite.

Testing performance is easy enough, but measuring endurance is too time-consuming for a comparison like this one. That's unfortunate, because write endurance is an important consideration given the contenders. As data is written to NAND, the individual memory cells degrade, shrinking the range of voltages that can be used to represent data. This shrinkage is more problematic for TLC flash, which must differentiate between more values within that narrowing window.

Samsung doesn't publish an official endurance specification for the 840 EVO, but Crucial says the M500 can withstand 40GB of writes per day for five years, or 72TB total. Based on what we've seen in our ongoing SSD Endurance Experiment, Crucial's estimate is a conservative one. Thus far, we've written over 400TB to a collection of MLC-based drives, and they're all in excellent shape. I can count the number of bad blocks on one hand.

Our experiment doesn't include the M500 or the 840 EVO. However, it does feature Samsung's first-generation TLC drive, the 840 Series. That SSD showed its first bad blocks after 100TB of writes, and it's accumulated over 1300 of 'em through 400TB of writes. We also encountered unrecoverable errors while performing a data retention test at the 300TB mark.

TLC NAND clearly isn't as robust as the MLC alternative. That said, hundreds of terabytes is an awful lot of writes for a consumer-grade SSD. The drive in my primary desktop writes only about 2TB per year, so it would take me half a century just to get to 100TB.

SSDs employ advanced signal processing and error correction algorithms to make the most of their NAND's limited lifespan. Crucial also endows the M500 with an extra layer of protection inherited from Micron's enterprise-class products. Dubbed RAIN, this RAID-like redundancy scheme uses a portion of the flash to house parity information. If the user's data is compromised by a physical flash failure, it can be reconstructed with the parity information stored on the drive.

The flash reserved for parity is inaccessible to the user, which explains the M500's lower capacity. The drive actually has a terabyte of flash onboard. RAIN and overprovisioned spare area each take a slice, leaving only 894GB for the user.

Even though the 840 EVO doesn't have to worry about parity data, its TurboWrite cache monopolizes some of the flash. With the same 1TB total NAND capacity as the M500, the EVO delivers 932GB to the user. The 38GB difference works out to only 4%, but that's still enough to store a lot of applications and games—or one blockbuster title with high-resolution textures.