The democratization of SSDs has been an exciting trend to watch. Over the past few years, prices have tumbled dramatically, and performance has improved by leaps and bounds. SSDs have gone from prohibitively expensive luxuries to essential components for all but the most affordable PCs.
More recently, consumer-grade SSDs seem to have plateaued. Prices have stabilized and even crept upward slightly from where they were last year. We haven't seen much development on the controller front, though NAND makers continue to churn out chips based on ever-smaller lithography. Most new SSDs are little more than tweaked versions of existing models outfitted with the next generation of flash memory. To be honest, that's made writing about them a little dull.
When SanDisk first introduced us to its Extreme II SSD, I thought we were in for more of the same. My eyes rolled—but only once, because this isn't another cookie-cutter SSD. Like most solid-state drives, the Extreme II relies on an array of MLC NAND for storage. However, it also has a built-in flash cache based on SLC memory.
SLC NAND has higher write performance and better endurance than MLC flash, making it ideal for caching solutions. It's also popular in high-end server SSDs. With only one bit per cell, though, SLC NAND costs more per gigabyte than its two-bit MLC counterpart. The Extreme II attempts to blend the best of both worlds by using an SLC cache to improve performance and MLC mass storage to keep the pricing competitive. How's that for something different?
Introducing the nCache
Solid-state caches are nothing new, of course. They're employed in hybrids that combine flash memory and mechanical storage in a single drive. SSD caching is also available via dual-drive configurations that use software to marry separate solid-state and mechanical drives. Unlike those implementations, the SanDisk Extreme II doesn't have a mechanical component; its so-called nCache sits in front of solid-state storage. The cache is used quite differently than what we've seen from other solutions, as well.
Only three kinds of data are stored in the nCache. The highest priority is given to the metadata that map logical block addresses to physical NAND locations. These data are replicated from the drive's DRAM cache memory, and they're backed up again in main storage for extra redundancy. Flush commands are also stored in the nCache, while the remaining space is dedicated to what SanDisk characterizes as "small" random writes.
The nCache accumulates smaller writes at a higher speed than the rest of the SSD. These writes are bundled into larger blocks, which are then moved to the main flash. This collation is employed to bridge the widening gap between the typical request size and the block sizes used in flash memory. Most system writes are 4KB or less, while the block size of newer NAND has pushed beyond 1MB.
SanDisk says the nCache has a "little less" than 1GB of total storage capacity. Some of that will be consumed by the metadata, leaving even less available as a random write buffer. If the nCache is full, all incoming data are written directly to main storage.
A gigabyte of nCache doesn't sound like a lot, but keep its mission in mind. SanDisk isn't speculatively pre-loading OS and application data. The small writes collected by the nCache shouldn't take up too much space.
Perhaps the most intriguing thing about the nCache is where it lives. Instead of being housed on a separate chip, it's spread evenly across the onboard NAND chips. This configuration allows the cache to exploit all the memory channels built into the controller.
The Extreme II's 19-nm Toggle DDR flash is made by SanDisk, and it's a unique SLC/MLC hybrid. One portion of the flash is configured as SLC NAND with one bit per cell. The remainder crams two bits per cell in an MLC setup. SLC flash boasts higher random write performance than the MLC stuff, which is why it's reserved for the nCache. MLC NAND offers a higher storage density, making it better suited for mass storage. Combining the two in a single chip is pretty slick.
In addition to relying on hybrid NAND, the nCache requires custom firmware. There's no need for a specialized controller, though. The Extreme II uses an off-the-shelf Marvell 88SS9187 chip. This controller has eight parallel NAND channels and can address up to four individual dies per channel—typical specifications for modern SSD silicon.
SanDisk sets aside 7% of the Extreme II's flash capacity as overprovisioned spare area. Additional capacity is reserved for the nCache, knocking the available models down to 120, 240, and 480GB. Unlike other SSDs with those capacities, the Extreme II doesn't feature RAID-like die redundancy to guard against physical flash failures. It doesn't appear to support hardware-accelerated encryption, either.
|Capacity||Die config||Max sequential (MB/s)||4KB random (IOps)||Price||$/GB|
|120GB||16 x 64Gb||550||340||91,000||74,000||$130||$1.08|
|240GB||32 x 64Gb||550||510||95,000||78,000||$230||$0.96|
|480GB||64 x 64Gb||545||500||95,000||75,000||$450||$0.93|
Like most contemporary SSDs, the Extreme II is built with 64Gb (8GB) NAND dies. The controller can address four chips on each of its eight channels, making 32 dies the minimum for optimal performance. The 120GB model doesn't have enough NAND to deliver peak performance, but the 240 and 480GB variants do. Oddly, though, the larger of the two has slightly lower performance ratings. We've asked SanDisk to explain the difference and are awaiting a response.
Regardless of the capacity, the Extreme II SSD is rated for 80TB of total writes. The drive is also covered by a five-year warranty, a perk typically restricted to high-end drives. The Extreme is priced more like a mid-range model, though.
Users can monitor flash wear using the Extreme II's SMART attributes, which track total host writes in addition to a "life curve" that describes overall drive health. These variables can be monitored with third-party software or checked with SanDisk's SSD Toolkit. The Toolkit app is considerably more limited than the utilities shipped with Intel and Samsung SSDs, but it does offer a built-in firmware updater.
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