In the world of mechanical hard drives, the new 6Gbps Serial ATA specification means very little. This third-generation standard's claim to fame is a faster host interface capable of shifting bits at 600MB/stwice the speed of the old 3Gbps spec. However, that 300MB/s "SATA II" interface was hardly a bottleneck for traditional hard drives. Even Western Digital's latest VelociRaptor, which is the fastest mechanical drive that plugs into a Serial ATA port, can only sustain transfer rates up to 152MB/s. The 'raptor's cache didn't push much more than 236MB/s in our burst speed tests, either, giving the drive little chance of living up to its 6Gbps interface speed.
If you want to wring more than 300MB/s from a mechanical hard drive, you're going to have to combine several of them in RAID. Solid-state drive makers are actually faced with the same challenge. Individual flash chips don't necessarily offer superior sequential throughput to traditional hard drives, which means that SSDs seeking to maximize performance must distribute the load across numerous chips tied to multiple memory channels, effectively creating a multi-channel array within the confines of a single drive.
With the average 2.5" SSD capable of accommodating at least 16 flash memory chips (and even more with double-stacking), there's plenty of parallelized performance potential just waiting to be tapped. That said, we've yet to see an SSD exceed the limits of the 3Gbps SATA spec. Solid-state drive makers have even shown admirable restraint in not endowing their drives with unnecessary 6Gbps SATA connectivity. All but one of them, that is. Crucial's RealSSD C300 claims not just to support the new Serial ATA spec, but also to exploit it.
A long-time player in the memory industry, Crucial is better known for DIMMs than solid-state drives. The company's latest SSD is the first on the market with a 6Gbps SATA interface and the only one whose performance ratings exceed second-gen SATA's 300MB/s ceiling. Naturally, we had to test one for ourselves.
Marvell controller, take two
The RealSSD inherits its 6Gbps Serial ATA support from Marvell's 88SS9174 flash controller. This new design succeeds the 88SS8014 controller we tested in Plextor's PX-128M1S back in April. Marvell's first controller was limited to a 3GBps SATA interface, and more importantly, it didn't support the TRIM command built into Windows 7. The Plextor drive's performance suffered mightily as a result, but we're pleased to report that TRIM has made the cut for the new model. TRIM works in conjunction with Marvell's garbage collection routine, which runs in the background to reclaim flash pages marked as available by the command. The frequency with which garbage collection is performed depends on how the drive is being used and how much free capacity it has available.
With eight memory channels, the Marvell controller has twice the number available in Indilinx's popular Barefoot design but is two short of the ten channels Intel squeezed into its X25-series SSDs. Crucial claims the RealSSD can sustain a sequential read rate of 355MB/s when connected to a 6Gbps SATA interface. The drive's sequential read performance purportedly drops to 265MB/s when using a 3Gbps link.
Connecting the RealSSD to a 3Gbps SATA controller won't impede the drive's write performance, though. Crucial lists a sequential write speed of 215MB/s for 256GB flavors of the C300 and 140MB/s for 128GB models. There's no difference in sequential read speeds between the two capacities, but they do differ when it comes to random-4KB performance ratings. The larger of the two can purportedly push 60k IOps with 4KB random reads and 45k IOps with random writes, while the 128GB drive is limited to 50k IOps with reads and 30k for writes.
The lower-capacity model is the slower of the two because it has less internal parallelism than the 256GB drive. Both feature 16 flash memory chips split evenly between the two sides of the circuit board. However, the 128GB model uses 8GB flash chips that have two dies each, while the 256GB drive is populated with 16GB chips that have four NAND dies apiece. For those who are keeping score at home, that's 32 dies for the 128GB drive and 64 for the 256GB one.
Even with only eight memory channels in the controller, it's possible for all of the drive's NAND dies to be active at the same time. According to Crucial, having access to the additional dies is especially helpful when writing to the drive because it takes "much longer" to program a NAND die than it does to issue the necessary commands and load the NAND cache registers with the data that's set to be written.
Crucial is the consumer face of memory mogul Micron Technology, so it's no surprise the RealSSD's flash memory bears the Micron name. Like those found in most other recent SSDs, the MLC memory chips inside our 256GB RealSSD are fabricated using 34-nm process technology. The 34-nm flash lurking in modern solid-state drives usually conforms to the Open NAND Flash Interface (ONFI) 1.0 standard, but the RealSSD's flash chips are hip to the much more recent ONFI 2.1 spec.
Among other things, the ONFI standard governs the speed of the NAND interface. Version 1.0 was limited to 50MB/s, while its 2.0 successor more than doubled the interface bandwidth to 133MB/s. ONFI 2.1 adds support for speeds of 166 and 200MB/s. The 2.1 spec also aims to improve read throughput with deeper pipelining and interleaving support.
Unfortunately, beyond listing support for ONFI 2.1, Micron's website doesn't provide any information on just how fast the RealSSD's NAND components, marked MT29F128G08CKCBBH2, might be. Odds are they can do better than 50MB/s, but I'd be surprised if they can push more than 133MB/s given the fact that Micron VP of Memory System Development Dean Klein only mentioned second-gen ONFI support when the C300 was first introduced.
Flipping the C300's circuit board reveals a DDR3 memory chip that serves as the drive's cache. The chip weighs in at a hefty 256MB, which is several times the capacity used in most SSDs. Kingston's Toshiba-based SSDNow V+ was the previous cache king with 128MB. Indilinx-based drives typically have 64MB caches, while Intel's X25 series makes do with a measly 32MB. Crucial uses the same 256MB cache on 128 and 256GB versions of the RealSSD.
Like most solid-state drives on the market, the C300 is covered by a three-year warranty. Three-year warranties are common for consumer-grade hard drives, too, but premium and enterprise-oriented models usually get five years of coverage. Given their premium pricing, solid-state drives surely qualify as premium products. It would be nice if they got some additional warranty coverage to match.
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