Single page Print

Intel's X25-M solid-state drive


The chip giant dips into the storage world
— 10:00 AM on September 8, 2008

Depending on who you believe, the solid-state storage revolution is on the horizon, just over it, a few years away, or already upon us. Analysts, journalists, and forum fanboys can't seem to come to an agreement on exactly where we are in the process. One thing seems certain, though: At least for some markets, it's not a question of if solid-state storage will take over from traditional mechanical hard drives, but when.

Of course, the mechanical hard drive is by no means dead. The torrid pace of platter development is still yielding impressive increases in performance and capacity. But as the price of solid-state alternatives tumbles, SSDs are sure to carve out a niche in the mobile space, where their superior shock tolerance and low power consumption are more important than storage capacity. Solid-state drives won't just make their way into notebooks, though. The near-instantaneous seek time of flash memory offers tantalizing performance potential for any workload dominated by random I/O requests. SSDs aren't nearly as impressive when it comes to sequential transfers, but they're getting faster on that front, too, making them interesting options for high-end desktops, workstations, and servers.

So solid-state drives are going to be big. Very big. And everyone wants a slice of the pie. It's no wonder, then, that industry giant Intel is eager to get in on the action. The company is in a rather unique position to serve the SSD market, too, with expertise not only in the chip manufacturing capability essential to populating a drive with memory, but also in the storage controller design that largely dictates performance.

Intel formally detailed its solid-state drive plans at IDF just a couple of weeks ago, revealing not just a single drive, but an entire collection of products that will trickle out in the coming months. The first of these SSDs to hit the market will be the X25-M, which boasts an impressive 250MB/s sustained read rate, a 70MB/s sustained write rate, 80GB of storage capacity in a 2.5" form factor, and support for Native Command Queuing. Read on to see how this drive stacks up against a mix of solid-state and mechanical alternatives through our exhaustive suite of performance, noise level, and power consumption tests.


Flash inside
The X25-M is based on multi-level cell (MLC) NAND flash memory—a cheaper alternative to the single-level cell (SLC) memory used in some flash drives. Cheaper is relative, of course; we're still talking about SSDs.

MLC memory is characterized by relatively slow write speeds, and the X25-M's 70MB/s sustained write rate lives up to MLC's reptuation. Calling 70MB/s relatively slow seems almost comical given that few 7,200-RPM notebook hard drives can match that speed. However, the Intel drive's whopping 250MB/s sustained read rate quickly puts things into perspective. That's more than twice the sustained read throughput of the latest desktop WD VelociRaptor and leagues ahead of the fastest mobile competition. (For comparison, the SLC-based 64GB Samsung FlashSSD we reviewed not long ago is rated for 100MB/s sustained reads and 80MB/s writes.) Finally, we have a drive capable of exploiting its 300MB/s Serial ATA interface.

While we're embarassing the VelociRaptor, we should note that the X25-M has a read latency of just 85 microseconds. That's 0.085 milliseconds, if we convert to units more commonly associated with hard drives. For comparison, the random access time of the VelociRaptor is 7.4 milliseconds—a difference of two orders of magnitude.


The X25-M naked, with its memory chips and storage controller exposed

The X25-M owes its low latency to the zippy access times inherent to flash memory. 20 flash chips can be found on the X25-M: 10 on the top of the drive's circuit board and 10 on the back, for a total of 80GB. Intel fabricates these chips itself using a 55nm process, although rival flash giant Samsung amusingly makes an appearance with a K4S281632I-UC60 memory chip that weighs in at 16MB. Way to throw Samsung a few dollars (or more likely, pennies) for every SSD sale you beat them to, Intel.

As one might expect from a company with a long history of developing core-logic chipsets, the X25-M's storage controller is an Intel design—and a smart one at that, with support for, ahem, SMART monitoring. More interestingly, the controller supports Native Command Queuing (NCQ)—a new trick for SSDs. NCQ was developed to reduce the performance impact of mechanical latency found in traditional hard drives, so it's might seem like an odd choice for a solid-state drive with no mechanical parts. According to Intel, its SSDs are so fast that NCQ helps to compensate for latency encountered in the host PC. Even today's fastest systems take some time (time is relative in the microsecond world of the SSD) between when a request is completed and another one is issued. Queuing up multiple requests can keep a solid-state drive busy during this downtime, and the X25-M is capable of stacking requests 32 deep.


More memory chips on the back

So the X25-M shouldn't be short on performance, but what about longevity? MLC-based flash memory cells are limited to 10,000 write-erase cycles, giving solid-state drives a finite lifespan. When estimating the operating life of their drives, other SSD makers generally rely on a basic formula to calculate the number of cycles used:

Cycles = (Host writes) / (Drive capacity)

Intel says this formula oversimplifies the issue, and that two other factors must be considered. The first of these variables is write amplification, which refers to the amount of data actually written to a drive for a given write request. Intel gives an example in which a host system generates a 4KB write request that, thanks to a drive's 128KB erase block size, actually incurs a 128KB NAND write. Dividing the NAND write size by the request size yields the amplification factor, which is 32 in this case. Intel says the X25-M's write-amplification factor is extremely low at 1.1, while "traditional" SSDs have much higher amplification factor of 20.

The efficiency of wear-leveling algorithms also has a hand in determining an SSD's lifespan. If a drive is going to shuffle bits around to avoid bad cells and more efficiently use those available, it must do so without wasting precious write-erase cycles. Intel estimates the X25-M's wear-leveling efficiency factor at less than 1.1, claiming that traditional SSDs have an efficiency factor of 3.

Taking write-amplification and wear-leveling efficiency into account, Intel says the correct formula for cycling is as follows:

Cycles = (Host writes) * (Write amplification factor) * (Wear leveling factor) / (Drive capacity)

Using a write-amplification factor of 1.1 and a wear-leveling efficiency factor of 1.1, 20GB of write-erase per day for five years should consume only about 550 cycles on an 80GB X25-M. Using "traditional" SSD technology with an amplification factor of 20 and an efficiency factor of 3, the same write-erase load would use over 27,000 cycles. That's a huge difference, and to be fair, it's one that relies on values provided by Intel that aren't entirely consistent. Another Intel presentation from IDF estimates that "mediocre" SSDs have a write-amplification factor of 10 and a wear-leveling efficiency factor of 5, resulting in just under 23,000 cycles for our 20GB of write-erase per day example. That presentation also pegs the X25-M's efficiency factor at 1.04 rather than 1.1. We can't easily test a drive's lifespan ourselves, but we did ask Samsung for the write-amplification and wear-leveling efficiency factor values for its SSDs. Samsung hasn't responded yet, though.

If you don't want to crunch through the math, Intel estimates that the 80GB X25-M will last for five years with "much greater than" 100GB of write-erase per day. That's a relatively long time for much more data than most folks are likely to write or erase on a daily basis.

Actual drive lifespans aside, Intel rates the X25-M's Mean Time Between Failures (MTBF) at 1.2 million hours. That's competitive with the MTBF rating of other MLC-based flash drives and equivalent to common MTBF ratings for enterprise-class mechanical hard drives.


A simple black casing holds everything together

Rather than banking on a single SSD, Intel has prepared a full lineup of solid-state drives with multiple form factors, capacities, and memory types. The X25-M will come first, with 80GB models in mass production this month, followed by 160GB derivatives early next year. Intel will also offer 1.8" versions of this drive destined for thin-and-light notebooks and, dare we dream, netbooks as well.

Lest one market segment go without unwarranted Mountain Dew branding, Intel has an Extreme SSD in the works, too. The X25-E will only be available in 2.5" form factors with capacity points at 32 and 64GB. This SLC-based drive maintains the X25-M's 250MB/s sustained read rate, but boosts writes up to 170MB/s and drops read latency to 75 microseconds. The Extreme also features an MTBF rating of two million hours and enjoys SLC memory's 100,000 write-erase cycle tolerance, making it even more attractive for enterprise environments. 32GB versions of the X25-E should enter production in the next three months, followed by 64GB flavors next year.

The X25-E certainly won't be cheap—SLC-based drives rarely are—but we don't yet know exactly how much it will cost. Intel has, however, confirmed that the 80GB X25-M will sell for $595 in 1,000-unit quantities. Drives will ship to the channel and major PC builders this week, and will either be sold on their own or installed in pre-built systems. We've also heard word that Intel partners may sell the company's SSDs under their own names. That wouldn't be surprising considering that we've already seen OCZ repackaging Samsung's FlashSSD drives.