Single page Print

Intel's X25-E Extreme solid-state drive

Now with single-level cell flash memory
— 10:25 PM on November 23, 2008

Manufacturer Intel
Model X25-E Extreme 32GB
Price (Street)
Availability Now

Intel burst onto the storage scene back in September with its X25-M solid-state drive. This 80GB SSD combined Intel's formidable chip-making prowess with its years of experience in high-performance storage controller design, and the results were impressive to say the least. Indeed, the X25-M was arguably the fastest solid-state drive we'd ever tested, let down only by the comparatively slow write performance of its multi-level cell (MLC) flash memory chips.

There isn't much Intel can do about the slow write speeds inherent to MLC memory. However, the company has crafted a new solid-state drive based on single-level cell (SLC) memory chips that aren't plagued by poor write rates. This latest X25-E Extreme boasts the same 250MB/s sustained read speed as the X25-M, but write speeds have been boosted from a paltry 70MB/s to a much more impressive 170MB/s. Oh my.

Obviously, the X25-E Extreme is going to be faster than the X25-M. Read on to see where the X25-E's faster write speeds help the most, and in some cases, where they improve performance more than you might expect.

Extreme for enterprise
Solid-state drives use either single-level or multi-level cell flash memory. The former stores one bit per memory cell (a value of 0 or 1) while the latter is capable of storing two bits per cell (with possible values of 00, 01, 10, and 11). Obviously, MLC flash has a significant advantage on the storage density front. However, that advantage comes at the cost of write speeds, which are typically much slower than reads. Intel's MLC-based X25-M, for example, is capable of reading at up to 250MB/s, but its sustained write speed tops out at only 70MB/s. Single-level cell memory doesn't suffer such a great disparity between read and write speeds, as evidenced by the X25-E Extreme, which reads at up to 250MB/s and writes at up to 170MB/s.

Of course, the more balanced transfer rates offered by SLC memory don't come cheap. The X25-M 80GB is currently selling for $621 online, which works out to a seemingly exorbitant $7.76 per gigabyte. But that's nothing compared to the cost of the X25-E Extreme 32GB, which at $719 online, rings in at an even steeper $22.47 per gigabyte. Solid-state storage isn't cheap, and single-level cell implementations are about as expensive as SSDs get.

The prospect of shelling out three times as much per gigabyte for the X25-E is certainly daunting, but the drive does offer other perks to justify the premium. For example, its 75-microsecond read latency is 10 microseconds quicker than that of the X25-M. That's not a huge margin, but within the confines of a modern PC, where bits flip at billions of times per second on multiple processor cores, it's a notable improvement. To put things in perspective, it's also worth noting that the access time of a VelociRaptor, which has faster seek times than any other Serial ATA hard drive, is two orders of magnitude slower at 7400 microseconds.

While the X25-E's faster write speed and quicker access latency are great, it's on the longevity front that the drive offers the biggest step up over the X25-M. Multi-level memory cells are limited to 10,000 write-erase cycles before they burn out. Single-level memory cells, on the other hand, are good for 100,000 write-erase cycles—a difference of one order of magnitude.

SLC-based flash drives should last much longer than those that use MLC memory chips. Exactly how much longer depends on several factors, including the write-erase content of the workloads involved, the size of the drive, its write amplification factor, and the efficiency of its wear-leveling algorithms. Solid-state drive makers tend not to discuss those last two factors, but according to Intel, they can have a profound impact on a drive's actual lifespan.

Write amplification refers to the amount of data that must actually be written to a drive to complete a given write request. Say you have a 4KB write request and a drive with a 128KB erase block size. You can't just erase and re-write 4KB of that 128KB erase block—you have to clear and rewrite the whole thing. The write amplification factor is the actual write size divided by the request size, which in this case is 32. Intel claims that the X25-E Extreme's write amplification factor is less than 1.1, and that "traditional" SSDs have a write amplification factor of closer to 20.

Wear-leveling is also an important component of flash endurance, as drives spread the love in an attempt to distribute write-erase cycles evenly across available cells. This requires some bit shuffling, and drives must take care to ensure that their wear-leveling algorithms don't burn through too many write-erase cycles in the process. According to Intel, most SSDs have a wear leveling efficiency factor of three. The X25-E Extreme's wear leveling efficiency factor is quoted as less than 1.1.

If we combine all the factors that Intel says affect solid-state drive longevity, we come up with the following formula for cycling:

Cycles = (Host writes) * (Write amplification factor) * (Wear leveling factor) / (Drive capacity)
With write amplification and wear leveling efficiency factors of 1.1, and 20GB of write-erase requests per day for five years, we should only burn through 1380 cycles on the X25-E Extreme. The same workload on what Intel defines as a "traditional" SSD, with a write amplification factor of 20 and a wear-leveling efficiency of three, consumes more than 68,000 cycles. We don't want to rely too much on Intel's likely pessimistic assessment of the wear leveling efficiency and write amplification factors of other solid-state drives, but other SSD makers haven't been able to give us equivalent numbers of their own.

The X25-E Extreme's expected lifespan will, of course, depend on how many gigabytes of write-erase operations are thrown at it. Even with 100GB of write-erase per day, it'll take more than 72 years to burn through the drive. Couple that with the Extreme's two-million-hour Mean Time Between Failures (MTBF) rating, and one can probably expect the drive to last.

Like its MLC-based cousin, the X25-E uses a 10-channel storage controller backed by 16MB of cache. Amusingly, the cache is provided by Samsung—one of the biggest players in the SSD market—via a K4S281632I-UC60 SDRAM memory chip. The storage controller is an Intel design that's particularly crafty, supporting not only SMART monitoring, but also Native Command Queuing (NCQ). NCQ was originally designed to compensate for the rotational latency inherent to mechanical hard drives, but here it's being used in reverse, because Intel says its SSDs are so fast that they actually encounter latency in the host system. It takes a little time (time is of course relative when you're talking about an SSD whose access latency is measured in microseconds) between when a system completes a request and the next one is issued. NCQ is used to queue up to 32 requests to keep the X25-E busy during any downtime between requests.

Even with its protective shroud removed, the Extreme looks not unlike the X25-M that preceded it. Both drives use a single circuit board populated with 10 memory chips on each side. Intel makes these chips itself using a 50nm fabrication process. With the X25-E, however, the connection points are covered with what appear to be drippings from World of Goo. No doubt this protective coating has been used to prevent enterprising pirates from, er, installing mod chips, or something.

Thus far, we've only touched on the performance benefits that solid-state hard drives can provde, but there are other advantages to moving out of the mechanical world. With no moving parts, SSDs are much more resistant to physical shock. They're absolutely quiet, too, and typically consume much less power than traditional hard drives. Intel rates the X25-E's idle power consumption at just 0.06W, and when active, that figure only jumps to 2.4W.

The X25-E's paltry power consumption will be particularly attractive for the enterprise applications at which the drive is targeted. Indeed, this may be the first enterprise-class product to bring Extreme branding into rack servers. Dude, that's totally where the X25-E's power savings will add up, as stacks of drives are combined in RAID arrays where every watt saved will also lower cooling costs for the rack.