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A fresh look at storage performance with PCIe SSDs

New benchmarks for the next storage revolution

For years, SSDs thrived under the Serial ATA banner. New generations got bigger, faster, and cheaper. Prices continue to fall and densities continue to increase, but performance has been stagnating for a while. The problem is the SATA 6Gbps interface, whose limited bandwidth keeps modern flash storage from living up to its true potential.

Fortunately, SSDs aren't inexorably tied to Serial ATA. They can tap into PCI Express, which promises substantially more bandwidth through faster signaling and multiple lanes. They can also ditch Serial ATA's dated AHCI protocol for an all-new NVM Express protocol architected with solid-state storage in mind.

PCIe SSDs have actually existed for a while, but most early implementations used bridge chips tied to the very same controllers found in SATA drives. Those initial efforts are very different from the native solutions that have crept into the market over the past year. This new breed employs updated controllers that meld PCI Express and NAND interfaces on a single chip. Native connectivity promises to elevate SSDs to new heights, setting the stage for the next revolution in PC storage.

To prepare for the incoming tide of PCIe SSDs, we've revamped our storage test rigs with new hardware and benchmarks. We've gathered a boatload of data on three PCIe drives and a stack of SATA SSDs, and we've learned some interesting lessons along the way. In some respects, the next PC storage revolution is already upon us. In others, though, things haven't changed one bit.

All aboard the PCI Express
Before we dive into our performance results, let's get acquainted with our posse of PCIe drives: Plextor's M6e 256GB, Samsung's XP941 256GB, and Intel's DC P3700 800GB. The M6e is the most consumer-friendly solution, so that's the best place to start.

Released last year, the M6e was the first native PCIe SSD to be widely available to end users. The double-sided M.2 2280 "gumstick" is a perfect fit for the mini SSD slots in most 9-series Intel motherboards, as is the dual-lane Gen2 interface. For systems that lack M.2 connectivity, Plextor sells versions of the M6e mounted to PCIe expansion cards that plug into full-sized slots.

Because it's a standard AHCI device, the M6e doesn't require separate drivers. Any modern operating system should recognize it. The M6e also works with both UEFI and legacy BIOSes, which should ensure broad compatibility with new and old motherboards alike. Boot support is pretty much guaranteed.

Marvell provides the M6e's controller, while Toshiba kicks in the 19-nm MLC NAND. Plextor claims this pairing is good for sequential reads up to 770MB/s, a fair bit faster than SATA's top speed. The fastest incarnation of the M6e is only rated for 625MB/s sequential writes, though, and our 256GB sample tops out at just 580MB/s. The M6e's ~100k IOps random I/O ratings aren't anything to write home about, either. Plenty of SATA drives boast similar specs.

Samsung's XP941 shares the same M.2 2280 form factor as the M6e, but only one side of its circuit board is populated. Unlike the Plextor, it was never targeted directly at consumers. Samsung designed the XP941 specifically for notebook makers, and the drive is a somewhat less natural fit for desktops as a result.

The XP941's homegrown controller uses the AHCI protocol, so at least OS support isn't an issue. However, booting from the drive requires UEFI-compatible firmware equipped with the appropriate option ROM. Notebook makers can easily integrate the necessary code into systems that use the XP941. So can mobo makers, but they have less incentive to support a drive that's rare even in PC enthusiast circles. For what it's worth, our test rigs' Asus Z97-Pro motherboards have no issues booting from the XP941.

The Samsung controller's four-lane Gen2 interface provides another clue that the XP941 wasn't meant for desktops. This wider pipe offers double the bandwidth available to the M6e, but it also requires more PCIe lanes than are available in most M.2 slots. Fortunately, four-lane adapter cards are capable of mating the XP941 with full-sized PCIe slots. The XP941 works in dual-lane M.2 slots, too, just not at full speed.

Speaking of which, the XP941 is rated to hit up to 1170MB/s with sequential reads and 930MB/s with writes. The 256GB version we've been testing is specced at 1080MB/s and 800MB/s, respectively, which still gives it a substantial leg up on the M6e. Random reads clock in at an impressive 120k IOps, but random writes are rated for half that—and only 72k IOps for the top model. Even Samsung's budget-minded 850 EVO SSD has better specs on that front.

Both of the M.2 drives are nearly a dollar per gig: the XP941 256GB sells for $229.99 at Newegg, while the equivalent M6e runs $215.99. That's peanuts compared to the ringer we drafted to illustrate the true potential of PCIe SSDs. Intel's DC P3700 800GB SSD costs over $2,400, or three bucks a gig, putting this datacenter-grade SSD on a whole 'nother level.

The P3700 is way too much SSD for an M.2 gumstick. It's only available in a half-height expansion card, as pictured above, or in an extra-thick 2.5" enclosure. Both implementations have substantial heatsinks, and the hunks of finned metal aren't just for show. The 800GB drive has an 18W power rating, while the 2TB flagship consumes up to 25W.

Intel evidently puts all that power to good use. The P3700 is rated to hit 2800MB/s in sequential reads and 1900MB/s in writes. Random reads peak at an astounding 460k IOps, according to the spec sheet, while random writes top out at an impressive 175k IOps (and 90k IOps for the 800GB unit we tested). SSD makers use different metrics to rate their drives, so don't get too caught up comparing the claimed numbers. The important takeaway is that the P3700 is a very different class of PCIe SSD.

The drive even features a different class of host interface. Intel's custom controller features a four-lane Gen3 interface with twice the bandwidth of an equivalent Gen2 link. On top of that, the chip conforms to the NVM Express protocol rather than AHCI. Otherwise known as NVMe, this protocol has much lower overhead than AHCI, and it's built to scale up with massively parallel flash arrays. Where AHCI is limited to a single command queue 32 entries deep, NVMe supports up to 64k parallel queues, each of which can be up to 64k slots deep.

Watch our discussion of PCIe SSDs on the TR Podcast

NVMe support is baked into Windows 8.1, and a hotfix is available to add support to Windows 7. Third-party drivers aren't required for those operating systems as a result. Intel still provides its own drivers for the P3700, and it claims they're faster than the ones that come with the OS. We've been using the Intel drivers for all our testing.

Native OS support makes it easy to get NVMe SSDs working as secondary storage. Booting from these drives requires UEFI firmware that explicitly supports NVMe boot devices, though. Support is a little spotty, as far as I can tell, and the Z97-Pro only received the necessary firmware update in February. Your mileage may vary with other makes and models.

With the PCIe drives covered, it's time to introduce the cadre of SATA SSDs we've gathered to provide comparative context. The field includes a diverse assortment of drives from some of the biggest names in the business: Crucial's BX100 and MX200, Intel's 335 and 480 Series, and Samsung's 850 EVO and 850 Pro. For nostalgia, we've also tested a vintage Intel X25-M G2 SSD that dates back to 2009. That drive is so old that its SATA interface is capped at 3Gbps speeds, but don't rule it out entirely. Turns out the old girl still has a few tricks up her sleeve.

Now, on to the benchmarks...