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Into Adaptive Memory
Seagate refers to the intelligence that populates the Momentus XT's flash as Adaptive Memory, cleverly disguising what is ultimately a massive read cache. Like ReadyDrive, Adaptive Memory populates the flash with data from the disk. However, Adaptive Memory is the only mechanism that writes to the flash. Writes from the host system never touch the flash memory, instead proceeding to the disk through the drive's DRAM cache, just like in a traditional hard drive.

Rather than looking at user access patterns at the file level, Adaptive Memory tracks Logical Block Address (LBA) access patterns to determine what gets cached. Access frequency is considered by the algorithm, but surprisingly, it's not the most important factor. Seagate maintains that not all data can be accessed significantly faster from the flash than it can be pulled from the platters. Flash may be substantially quicker than mechanical media with small random reads, but the gap between the two is much tighter with larger sequential transfers, especially if you don't have a lot of flash chips to spread across a multi-channel array. That's why Adaptive Memory puts only the data it thinks will deliver the biggest performance gains into the flash. When the flash is filled to capacity, access frequency becomes more of a determining factor.

Naturally, it takes some time for Adaptive Memory to learn a user's tendencies. Seagate says that a task must be performed two to three times before Adaptive Memory has the flash's contents optimized for it. Adaptive Memory is a relatively quick study, then, but also one that's prone to memory loss. Because it reorganizes data on the disk, defragmenting the Momentus XT will reset everything that Adaptive Memory has learned.

4GB of customized Micron SLC NAND flash

Adaptive Memory caches data in the Momentus XT's flash in chunks as small as 2KB. That granularity nicely matches the flash's custom formatting, which uses a 2KB page size instead of the 4KB pages found inside most SSDs. Each flash block still has 128 pages, but because they're smaller, the block size is only 256KB as opposed to 512KB. This smaller block size should lessen the impact of the block-rewrite penalty by reducing the size of a single block rewrite. That said, the block-rewrite penalty is really a non-issue for the XT because the host system is never waiting on a flash write. The block-rewrite penalty will only affect how quickly Adaptive Memory can transfer data between the platters and the flash.

Unlike most solid-state drives, which employ a multitude of multi-level cell (MLC) flash modules, the Momentus XT makes do with a solitary single-level cell (SLC) chip that weighs in at 4GB. SLC memory is common in enterprise-class SSDs, where its order-of-magnitude advantage in write-erase endurance over MLC flash offers greater longevity. Obviously, Seagate doesn't want Adaptive Memory to burn too quickly through the 10,000 write-erase cycles that typical MLC chips can handle.

SLC flash also tends to have faster write speeds than MLC. Seagate says that the XT's flash has peak reads speeds of 140MB/s and peak write speeds of 100MB/s. That's pretty spry for a single chip.

The XT's flash memory is joined by a 32MB DRAM chip that offers twice the capacity of the cache used in Seagate's previous 7,200-RPM mobile flagship, the Momentus 7200.4. The capacity bump is supposed to speed write performance, although it's likely a way for Seagate to differentiate the XT from more mainstream Momentus drives, as well. On the desktop, the Barracuda XT has twice the cache of other 'cudas.

All the hybrid's silicon elements litter the underside of the drive's PCB

A single Seagate drive controller governs the XT's cache, flash, mechanical platters, and host interface. Seagate elected to stick with a 3Gbps Serial ATA interface, which makes perfect sense given the drive's mobile ambitions. Besides, the XT isn't fast enough to require more than a 300MB/s SATA link.

On the mechanical front, the XT features a pair of 250GB platters spinning at 7,200 RPM. That's the same platter configuration as the Momentus 7200.4 we reviewed a year ago, so it's not exactly cutting-edge technology. Indeed, Seagate has already announced a new 750GB mechanical Momentus model that packs two 375GB platters with the same 7,200-RPM spindle speed as the XT. Seagate says a different design team was responsible for the XT, which is why that drive isn't using the new platters.

With a much higher areal density, Seagate's 375GB platters should be capable of sustaining higher read and write speeds than the 250GB disks inside the Momentus XT. We were supposed to receive one of the new Momentus 750GB drives with our XT sample, but we've now been told that the high-capacity drive won't arrive until mid-June. Perhaps Seagate wanted to a avoid having its new mobile flagship compared with a drive that will almost certainly offer superior sequential throughput.

The XT's platters are claimed to be capable of hitting speeds of up to 153MB/s, nicely illustrating Seagate's rationale for not caching everything in the flash, which reads at 140MB/s. Seeks to the flash will be nearly instantaneous, while it'll take an average of 12 ms to seek data off the disk. A dozen milliseconds may sounds nearly instantaneous, too, but keep in mind that hard drives live in a world where multiple processor cores tick away at billions of cycles per second.

The PCB otherwise hides the XT's Adaptive Memory components from view

You'll notice that I haven't spent any time talking about Adaptive Memory's advanced power-saving features. That's because Seagate is using the flash primarily to improve performance rather than to reduce power consumption. The drive will spin down the platters to a standby mode after a minute of inactivity. If a read request can be completely serviced by the contents of the flash memory while the drive's in standby mode, there's no need to spin up the platters. However, a missed read or any write operation will kick the platters into gear again. Adaptive Memory won't cache data based on whether having it in the flash is likely to extend the time the platters spend in standby mode.

Then again, the Momentus XT doesn't look to be particularly power hungry. The drive's power consumption is rated at just 1.6W while seeking and half that at idle. We'll see how the drive's real-world power draw measures up a little later in the review.

Although our focus today is on the 500GB Momentus XT, the drive is available in 320 and 250GB capacities, as well. Seagate's official suggested retail prices for the 500, 320, and 250GB drives are $156, $132, and $113, respectively. Street prices are already lower, with Newegg listing the 500GB drive at $130 and Amazon selling the $320GB for $116 and the 250GB for $108. Even at those prices, you're looking at a hefty mark-up over the Momentus 7200.4, whose 500GB flavor can be had for only $90.

We'll see how the XT's performance compares to that of the 7200.4 in a moment. First, I should point out that the XT gets two years more warranty coverage than standard Momentus drives and most other mobile hard drives on the market. Consumer-grade hard drives and the majority of SSDs are typically covered with three-year warranties. Seagate reserves five years of coverage for its enterprise-class products and flagship XT models, like this one.