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Seagate’s Momentus XT 750GB hybrid hard drive

Geoff Gasior
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For most folks, hybrids are probably the future of PC storage. Flash memory’s near-instantaneous access times are a performance benefit too good to pass up. At the same time, NAND flash’s relatively high cost per gigabyte, combined with our ever-growing thirst for storage capacity, ensures mechanical platters will remain in a supporting role for the foreseeable future. Music and movie archives aren’t likely to benefit from the wicked-fast access times of solid-state storage, anyway.

On the desktop, cobbling together a hybrid solution is as easy as putting a hard drive and an SSD in the same system. Users are free to manage the distribution of data between the two or to rely on third-party software to treat the SSD as a cache for the hard drive. Those approaches work just as well in the mobile world, but only in notebooks that support dual drives, which are relatively rare outside hulking desktop replacements.

For much of the notebook market, the only way to enjoy a hybrid storage configuration is with a single drive that combines flash and mechanical components inside the same 2.5″ form factor. Seagate has been making such drives for several generations now, and it claims to have sold more than a million of ’em over the years. Most of those hybrids hailed from the original Momentus XT family, which debuted last summer.

Unlike Seagate’s previous hybrid efforts, which relied on the ReadyDrive caching mechanism built into Windows Vista, the Momentus XT took matters into its own hands with an OS-independent caching scheme run entirely within the drive itself. The end result melded 500GB of mechanical storage with 4GB of flash memory dedicated to caching frequently accessed data. Today, that first-generation Momentus XT is being replaced by a new model that combines higher-capacity platters with faster flash memory—and 8GB of it. We’ve taken the new hybrid for a spin to see if it can hang with not only its mechanical competition, but also the droves of solid-state drives that have flooded the market over the past year.

Momentus XT, take two
Like its predecessor, the new Momentus XT uses its flash memory solely as a read cache. Incoming writes from the host bypass the flash completely. The only time data is written to the flash is when the drive’s caching scheme, dubbed Adaptive Memory, is populating it with data from the mechanical platters. Third-party caching solutions like Intel’s Smart Response Technology allow flash storage to cache both reads and writes, and Seagate indicates it may move in that direction with its own hybrids. For now, though, the Momentus XT’s flash component will only be capable of speeding up the drive’s read performance.

Seagate’s Adaptive Memory caching tech targets frequently accessed data at the block level rather than the file level. To better take advantage of the new drive’s larger allocation of flash memory, the caching scheme has received a number of tweaks that Segate has wrapped up in a marketing name: FAST Factor. The most interesting of these tweaks may be the changes to how the caching system treats Windows files associated with the boot process.

Dubbed FAST Factor Boot, this new approach aims to accelerate the Windows boot process whether you’re firing up the OS for the first time or rebooting after months of uninterrupted activity. With the old Momentus XT, Adaptive Memory needed a few boots to learn which OS-related data to cache. Using the drive for long enough without rebooting had the potential to push that data out of the cache, reversing any previous boot-time benefits. With the new XT, a segment of the NAND has been reserved solely for OS data related to the boot process. That section of the flash is populated as Windows is installed to the drive, which should speed things up starting from the first boot. If Windows is being installed via a drive image, the XT will need a couple of boots to determine what to put in the roped-off section of its cache. As with clean installs, that data won’t be kicked out of the cache between boots.

On our sample drive, the cache is an 8GB SLC NAND chip made by Micron. Seagate says it will be sourcing flash from multiple suppliers and that the chips could be configured with either 4KB or 8KB flash pages. The original XT uses smaller 2KB block sizes, but like the newer drive, it’s also based on SLC-type flash. SLC memory typically has 10 times the write-erase endurance of the MLC NAND chips found in consumer-grade SSDs, making it ideal for caching applications. The 311 Series SSD Intel designed for Smart Response Technology also uses SLC NAND, albeit 20GB of it.

The Momentus XT’s 8GB cache is still double the amount of flash available in the original drive. The NAND is faster, too. Seagate claims the new XT can read from its cache at 180MB/s, a 40MB/s improvement over the first model. Both hybrids can purportedly transfer data from the platters to the flash at 100MB/s.

Those speeds might not sound all that impressive in the context of modern SSDs, but keep in mind that the Momentus XT has only a single flash chip. Seagate says it’s using a custom bridge design to extract as much performance as possible from that solitary chip. However, the Momentus doesn’t have anywhere near the parallelism of modern SSDs, which use multi-channel controllers to address flash arrays typically made up of 8 to 16 NAND chips.

The only other memory on the Momentus XT is a 32MB Samsung DRAM chip that’s used as a traditional drive cache. The DRAM chip isn’t any bigger than the one used on the original Momentus XT, but Seagate says the new drive’s move to a 6Gbps Serial ATA interface will give the cache a little more room to stretch its legs. The XT’s flash component isn’t quick enough to benefit from the faster SATA pipe.

Momentus XT G2 Momentus XT G1
SATA interface 6Gbps 3Gbps
Spindle speed 7,200 RPM 7,200 RPM
DRAM cache size 32MB 32MB
Flash cache size 8GB 4GB
Available capacities 750GB 500, 320, 250GB
Platter capacity 375GB 250GB
Areal density 541 Gb/in² 394 Gb/in²
Warranty length Five years Five years

We’ve spent a lot of time talking about the Momentus XT’s flash component, but there’s a hard drive lurking under the hood, too. The Momentus XT has dual 375GB platters spinning at 7,200 RPM. Each platter has an areal density of 541GB/in², which is a 37% increase over the old model. Higher areal densities can improve performance by putting more data under the drive head with each revolution of the platter, so the new XT should offer a nice step up in performance even when its flash memory is going unused.

The original Momentus XT was available in multiple capacities, but Seagate is limiting the incoming drive to a single 750GB flavor. The old XT’s lower capacity points simply didn’t sell well, the company says. The original 500GB model will persist alongside the new hybrid, but there’s quite a difference in price between them. Just hours before its official product announcement, Seagate changed the price of the 750GB XT from $189 to $245. The old 500GB model currently sells for around $150 online.

Both drives are covered by a five-year warranty, which is considerably longer than the three years of coverage commonly attached to consumer storage products, SSDs included. Only Western Digital’s Black series of hard drives matches the XT’s five-year warranty.

The twins get a cooling upgrade
Our intrepid team of storage test systems received a rather major upgrade to Sandy Bridge hardware a few months back. More recently, we swapped their Thermaltake SpinQ coolers for a pair of much beefier Frio air towers.

These dual-fan monsters situate a pair of 120-mm spinners on either side of a heatpipe-infused radiator. The three-pin fans hang on rubber mounts to dampen vibration, and their speeds can be tweaked using a pair of in-line rheostats that offer fine-grained control. With both fans spinning at their slowest pace, the Frio is reasonably quiet. Crank them up to full tilt, and Thermaltake says the cooler will dissipate up to 220W—more than double the TDP of the fastest Sandy Bridge CPU.

Our testing methods
When briefing me on the Momentus XT, Seagate was quick to extol the drive’s virtues for desktop applications, citing Dell’s use of the old model in some of its business-oriented Optiplex systems. This isn’t just a drive targeted at mainstream notebooks, so we have no qualms about pitting it against a wide variety of competitors that include a 3.5″ Caviar Black desktop drive, a huge selection of the latest and greatest SSDs, and a handful of more direct 2.5″ mobile competitors. The chart below summarizes all of the particulars of the drives involved. I’ve greyed out the SSDs here and in the graphs on the following pages to make things more readable—and to clearly separate a different class of products.

Interface Cache Spindle speed Areal density Flash controller NAND
Corsair Force Series 3 120GB 6GBps NA NA NA SandForce SF-2281 25-nm Micron async MLC
Corsair Force Series 3 240GB 6Gbps NA NA NA SandForce SF-2281 25-nm Micron async MLC
Corsair Force Series GT 120GB 6GBps NA NA NA SandForce SF-2281 25-nm Intel sync MLC
Corsair Force Series GT 240GB 6GBps NA NA NA SandForce SF-2281 25-nm Intel sync MLC
Corsair Performance 3 Series 128GB 6GBps 128MB NA NA Marvell 88SS9174 34-nm Toshiba MLC
Crucial m4 128GB 6GBps 128MB NA NA Marvell 88SS9174 25-nm Micron sync MLC
Crucial m4 256GB 6Gbps 256MB NA NA Marvell 88SS9174 25-nm Micron sync MLC
Intel 320 Series 120GB 3GBps 64MB NA NA Intel PC29AS21BA0 25-nm Intel MLC
Intel 320 Series 300GB 3Gbps 64MB NA NA Intel PC29AS21BA0 25-nm Intel MLC
Intel 510 Series 120GB 6GBps 128MB NA NA Marvell 88SS9174 34-nm Intel MLC
Intel 510 Series 250GB 6Gbps 128MB NA NA Marvell 88SS9174 34-nm Intel MLC
Kingston HyperX 120GB 6GBps NA NA NA SandForce SF-2281 25-nm Intel sync MLC
OCZ Agility 3 120GB 6GBps NA NA NA SandForce SF-2281 25-nm Micron async MLC
OCZ Vertex 3 120GB 6GBps NA NA NA SandForce SF-2281 25-nm Intel sync MLC
Seagate Momentus 5400.4 25GB 3Gbps 8MB 5,400 RPM 204 Gb/in² NA NA
Seagate Momentus XT 500GB 3Gbps 32MB 7,200 RPM 394 Gb/in² NA* 4GB SLC
Seagate Momentus XT 750GB 6Gbps 32MB 7,200 RPM 541 Gb/in² NA* 8GB SLC
WD Caviar Black 1TB 6Gbps 64MB 7,200 RPM 400 Gb/in² NA NA
WD Scorpio Black 750GB 3Gbps 16MB 7,200 RPM 520 Gb/in² NA NA

The Momentus XT’s most direct competition comes from Western Digital’s Scorpio Black 750GB, another 2.5″ notebook drive with the same total capacity and 7,200-RPM spindle speed. There’s no flash memory inside the Scorpio, though, and it’s an older drive with less DRAM cache and a slower SATA interface. The Momentus XT 750GB’s hybrid competition will come from the original 500GB version of the Momentus XT.

Although its results are shown in all the performance graphs, we’re not too concerned with how the new XT fares against the Momentus 5400.4, a notebook drive that dates back to 2007. The 5400.4 will serve as our performance baseline for the value calculations that appear later in the review. It’s the slowest Serial ATA drive I could find in the Benchmarking Sweatshop.

We used the following system configuration for testing:

Processor Intel Core i7-2500K 3.3GHz
Motherboard Asus P8P67 Deluxe
Bios revision 1850
Platform hub Intel P67 Express
Platform drivers INF update 9.2.0.1030
RST 10.6.0.1022
Memory size 8GB (2 DIMMs)
Memory type Corsair Vengeance DDR3 SDRAM at 1333MHz
Memory timings 9-9-9-24-1T
Audio Realtek ALC892 with 2.62 drivers
Graphics Asus EAH6670/DIS/1GD5 1GB with Catalyst 11.7 drivers
Hard drives Corsair Force Series 3 120GB with 1.3 firmware
Corsair Force 3 Series 240GB with 1.3.2 firmware
Corsair Force Series GT 120GB with 1.3 firmware
Corsair Force Series GT 240GB with 1.3.2 firmware
Crucial m4 128GB with 0009 firmware
Corsair m4 256GB with 0009 firmware
Intel 320 Series 120GB with 4PC10362 firmware
Intel 320 Series 300GB with 4PC10362 firmware
Intel 510 Series 120GB with PPG4 firmware
Intel 510 Series 250GB with PWG2 firmware
Kingston HyperX 120GB with 320ABBF0 firmware
Corsair Performance 3 Series 128GB with 1.1 firmware
OCZ Agility 3 120GB with 2.15 firmware
OCZ Vertex 3 120GB with 2.15 firmware
WD Caviar Black 1TB with 05.01D05 firmware
Seagate Momentus 5400.4 250GB with 3.AAB firmware
Seagate Momentus XT 500GB with SD22 firmware
WD Scorpio Black 750GB with 01.01A01 firmware
Seagate Momentus XT 750GB with SM12 firmware
Power supply Corsair Professional Series Gold AX650W
OS Windows 7 Ultimate x64

Thanks to Asus for providing the systems’ motherboards and graphics cards, Intel for the CPUs, Corsair for the memory and PSUs, Thermaltake for the CPU coolers, and Western Digital for the Caviar Black 1TB system drives.

We used the following versions of our test applications:

Some further notes on our test methods:

  • To ensure consistent and repeatable results, the SSDs were secure-erased before almost every component of our test suite. Some of our tests then put the SSDs into a used state before the workload begins, which better exposes each drive’s long-term performance characteristics. In other tests, like DriveBench and FileBench, we induce a used state before testing. In all cases, the SSDs were in the same state before each test, ensuring an even playing field. The performance of mechanical hard drives is much more consistent between factory fresh and used states, so we skipped wiping the HDDs before each test—mechanical drives take forever to secure erase.

  • We run all our tests at least three times and report the median of the results. We’ve found IOMeter performance can fall off with SSDs after the first couple of runs, so we use five runs for solid-state drives and throw out the first two. The Hybrid drives have also been subjected to five runs, but only in tests that show their performance improving after the first one.

  • Steps have been taken to ensure that Sandy Bridge’s power-saving features don’t taint any of our results. All of the CPU’s low-power states have been disabled, effectively pegging the 2500K at 3.3GHz. Transitioning in and out of different power states can affect the performance of storage benchmarks, especially when dealing with short burst transfers.

The test systems’ Windows desktop was set at 1280×1024 in 32-bit color at a 75Hz screen refresh rate. Most of the tests and methods we employed are publicly available and reproducible. If you have questions about our methods, hit our forums to talk with us about them.

HD Tune — Transfer rates
HD Tune lets us present transfer rates in a couple of different ways. Using the benchmark’s “full test” setting gives us a good look at performance across the entire drive rather than extrapolating based on a handful of sample points. The data created by the full test also gives us fodder for a line graph.

To make the line graph more readable, we’ve excluded the SSDs. The solid-state drives have also been greyed out in the bar graphs to focus our attention—and to keep the graphs from looking like a technicolor mess. The rest of the drives have been color-coded by manufacturer, with the Momentus XT 750GB highlighted in a different shade to set it apart from the other Seagate models.

The Scorpio Black has slightly higher sequential read speeds than the Momentus XT 750GB. Still, the new hybrid is a vast improvement over the old XT. Read speeds on the old XT fall off more dramatically than they do on the other mechanical drives.

Of course, neither the mechanical drives nor the hybrids are within striking distance of the SSDs. Even the slowest member of the solid-state pack has double the average read speed of the Caviar Black 1TB. The fastest SSD reads data at a whopping four times the rate of the Caviar.

Switching to HD Tune’s write speed test doesn’t change the dynamic between our 750GB notebook drives; the Momentus XT can’t quite catch the Scorpio Black. The gap between the Momentus hybrids is much narrower here than it was with reads. So is the distance to the closest SSD, which is only barely faster than the best the mechanical field has to offer.

Although SSDs tend to write slower than they read, the best of the current crop is still well ahead of the Momentus XT. The Crucial m4 128GB offers more than double the average write speed of the XT, and the fastest solid-state drives achieve more than triple the write rate.

HD Tune’s burst speed tests are meant to isolate a drive’s cache memory.

Although the Momentus XT 750GB can’t sustain transfer rates that exceed the speed of a first-generation Serial ATA interface, short burst transfers will make use of the drive’s 6Gbps SATA link, if only just. The XT’s burst speeds are greater than those of its platter-bound compatriots—and even a few SSDs. This performance also represents quite a comeback versus the Scorpio Black, which is much slower with burst reads and writes.

HD Tune — Random access times
In addition to letting us test transfer rates, HD Tune can measure random access times. We’ve tested with four transfer sizes and presented all the results in a couple of line graphs. We’ve also busted out the 4KB and 1MB transfers sizes into bar graphs that should be easier to read. Once again, we’ve dropped the SSDs from the line graphs for reasons that should become obvious when you look at the results.

At transfer sizes up to 64KB, the Momentus XTs are clearly serving HD Tune’s random reads out of their respective flash caches. The new model is a little bit quicker, but both are in the same ballpark as the SSDs and orders of magnitude faster than the purely mechanical drives.

The 1MB transfer size proves too large for Adaptive Memory to handle, bringing the access times of the hybrids in line with those of their mechanical cousins. The Momentus XT 750GB is only a millisecond ahead of the Scorpio Black here, but the new XT has a good 4.5 ms on the old one.

Interestingly, the 750GB Momentus XT and the Scorpio Black both have slower access times with 512-byte random reads. Those drives use Advanced Format, which segments the platters in 4KB rather than 512-byte sectors. We’re using the latest version of HD Tune, which is optimized for 4KB sectors.

Through the 64KB transfer size, the Momentus XT 750GB’s access times are a little quicker than those of the Scorpio Black. That situation is reversed at the 1MB transfer size.

The largest transfer size drags the SSDs out of sub-millisecond territory, but they’re still substantially faster than the hybrids and mechanical drives. Let’s see how that advantage translates to more real-world tests.

TR FileBench — Real-world copy speeds
Concocted by resident developer Bruno “morphine” Ferreira, FileBench runs through a series of file copy operations using Windows 7’s xcopy command. Using xcopy produces nearly identical copy speeds to dragging and dropping files using the Windows GUI, so our results should be representative of typical real-world performance. We tested using the following five file sets—note the differences in average file sizes:

Number of files Total size Average file size
Movie 6 4.1GB 701MB
RAW 101 2.32GB 23.6MB
MP3 549 3.47GB 6.48MB
TR 26,767 1.7GB 64.6KB
Mozilla 22,696 923MB 39.4KB

The names of most of the file sets are self-explanatory. The Mozilla set is made up of all the files necessary to compile the browser, while the TR set includes years worth of the images, HTML files, and spreadsheets behind my reviews.

The Momentus XT 750GB lags behind the strictly solid-state drives in FileBench, but it’s consistently at the front of the non-SSD field. The XT is particularly proficient with the smaller files that make up the Mozilla and TR file sets, but I’m hesitant to credit Adaptive Memory. We tested the hybrids with five runs each, and their copy speeds didn’t improve by more than 1MB/s from the first run. Also, the original Momentus XT doesn’t fare that well with the Mozilla and TR file sets.

Whatever the reason, the XT delivers substantially faster copy speeds than the Scorpio Black. Even the 3.5″ Caviar Black desktop drive is put on the ropes by the hybrid.

TR DriveBench 1.0 — Disk-intensive multitasking
TR DriveBench allows us to record the individual IO requests associated with a Windows session and then play those results back as fast as possible on different drives. We’ve used this app to create a set of multitasking workloads that combine common desktop tasks with disk-intensive background operations like compiling code, copying files, downloading via BitTorrent, transcoding video, and scanning for viruses. The individual workloads are explained in more detail here.

Below, you’ll find an overall average followed by scores for each of our individual workloads. The overall score is an average of the mean performance score with each multitasking workload.

SSDs have always fared well in DriveBench, and they have a huge lead on the Momentus XT. Seagate’s newest hybrid remains comfortably ahead of the Caviar Black and the other mechanical drives, though. Let’s break down the individual workloads to see how things stack up.

With the exception of the copy workload, where the Momentus XT 750GB struggles mightily, the hybrid actually fares rather well. Each workload was run five times to let Adaptive Memory cache accordingly, and performance did improve through the first few runs. Even with those speedups, though, the Momentus XT is the slowest of the lot with the copy workload.

To be fair, the Momentus XT isn’t the only drive to suffer with one of the workloads. The Scorpio Black 750GB stumbles when we add video transcoding to our multitasking cocktail.

TR DriveBench 2.0 — More disk-intensive multitasking
As much as we like DriveBench 1.0’s individual workloads, the traces cover only slices of disk activity. Because we fire the recorded I/Os at the disks as fast as possible, solid-state drives also have no downtime during which to engage background garbage collection or other optimization algorithms. DriveBench 2.0 addresses both of those issues with a much larger trace that spans two weeks of typical desktop activity peppered with multitasking loads similar to those in DriveBench 1.0. We’ve also adjusted our testing methods to give solid-state drives enough idle time to tidy up after themselves. More details on DriveBench 2.0 are available on this page of our last major SSD round-up.

Instead of looking at a raw IOps rate, we’re going to switch gears and explore service times—the amount of time that it takes drives to complete an I/O request. We’ll start with an overall mean service time before slicing and dicing the results.

The Momentus XT 750GB makes a good first impression in DriveBench 2.0. While the solid-state drives remain firmly ahead, the hybrid has a healthy edge over everything else that has spinning platters.

Looking at the data split between reads and writes, it seems the Momentus XT 750GB’s flash cache is indeed getting a workout thanks to our two-week trace. The new hybrid outclasses its mechanical counterparts in reads, but it’s sandwiched between the Caviar and Scorpio Black in writes, which the flash can’t accelerate. Note the SSDs also fare better overall with reads than with writes.

There are millions of I/O requests in this trace, so we can’t easily graph service times to look at the variance. However, our analysis tools do report the standard deviation, which can give us a sense of how much service times vary from the mean.

We’re looking at consistency rather than raw performance here, but the Momentus XT 750GB is stuck in the same old pattern. The drive is in a better competitive position with reads than it is with writes. The Seagate drive does offer more consistent access times than the Scorpio Black on both fronts, though.

If I haven’t already scared you off with too many graphs and statistics, this next pair will do it. We’re going to close out our DriveBench analysis with a look at the distribution of service times. I’ve split the tally between I/O requests that complete in 0-1 milliseconds, 1-100 ms, and those that take longer than 100 ms to complete.

Here we have more evidence of the Momentus XT’s dual nature. The 750GB hybrid completes more reads in less than a millisecond than any other mechanical drive. For writes, however, it’s trumped by both WD drives.

At least the new Momentus XT is a clear upgrade over the old model. On the original Momentus XT, an awfully high percentage of both reads and writes take more than a millisecond to complete.

IOMeter
Our IOMeter workloads feature a ramping number of concurrent I/O requests. Most desktop systems will only have a few requests in flight at any given time (87% of DriveBench 2.0 requests have a queue depth of four or less). We’ve extended our scaling up to 32 concurrent requests to reach the depth of the Native Command Queuing pipeline associated with the Serial ATA specification. Ramping up the number of requests also gives us a sense of how the drives might perform in more demanding enterprise environments, for which the Momentus XT is certainly not designed.

The randomized access patterns of our IOMeter workloads play right into the strengths of solid-state drives, which are orders of magnitude faster than their mechanical counterparts—hybrids included. That performance discrepancy makes the graphs unreadable with the SSD results included, so we’ve again excluded them.

Whatever prevents the old Momentus hybrid’s transaction rates from scaling up in IOMeter hasn’t infected the new XT. The 750GB hybrid largely shadows the Scorpio Black, with both notebook drives turning in slower transaction rates than the desktop Caviar. Surprisingly, our performance baseline, the Momentus 5400.4, doesn’t fare as poorly as one might expect.

Boot duration
Before timing a couple of real-world applications, we first have to load the OS. We can measure how long that takes by checking the Windows 7 boot duration using the operating system’s performance-monitoring tools. This is actually the first time we’re booting Windows 7 off each drive; up until this point, our testing has been hosted by an OS housed on a separate system drive.

Even with our test system running off a disk image rather than a fresh install, the Momentus XT 750GB had no problem figuring out how to speed up the Windows 7 boot process. The drive’s first boot took nearly 15 seconds, but five of those seconds had been shaved off by the third iteration. We saw a similar improvement in boot times with the 500GB hybrid, which is still quite a bit slower overall. The old hybrid is quicker to load the OS than the Scorpio Black, though.

Impressively, the 750GB XT’s boot time brings it to within just two seconds of the fastest SSD. Let’s see if the hybrid can stay close when loading game levels.

Level load times
Modern games lack built-in timing tests to measure level loads, so we busted out a stopwatch with a couple of reasonably recent titles.

Adaptive Memory worked its magic in Duke Nukem Forever, but it didn’t have much of an impact in Portal 2. The latter loaded a second or two faster on the hybrids after the first run, while Duke load times were cut by 5-8 seconds.

Even without its flash cache hastening Portal 2 load times, Seagate’s latest hybrid is still faster than the Scorpio Black. As one might expect, the XT is more competitive with the solid-state field in Duke Nukem.

Noise levels
While some folks spent Black Friday stampeding through retail stores in search of deep discounts, I was holed up in the garage trying to spruce up our noise testing for hard drive reviews. We don’t have the space for anything elaborate, but I was able to whip up an 18″ x 20″ anechoic chamber on the cheap using 3/4″ MDF lined with acoustic foam. Say hello to the Box ‘o Silence, which is meant to isolate drives from the background noise of the Benchmarking Sweatshop.

Our TES-52 digital sound level meter slides into a hole drilled into one end of the box, and SATA power and data cables have been routed through another hole at the opposite end. Both holes are sealed tight with additional foam to prevent external sounds from seeping into the enclosure. To keep the lid nice and tight, elastic cord hooks onto anchor points at each corner.

The cord is leftover material from the suspension system designed to hold hard drives in place. Drives are suspended exactly 4″ from the sound meter’s foam tip and about the same distance from the box’s floor, ceiling, and side walls. To ensure the lowest possible ambient noise levels, we swapped the test system’s graphics card for a passively-cooled Gigabyte model and unplugged one of the Frio’s dual fans. Noise levels were measured after one minute of idling at the Windows desktop and during an HD Tune seek test.

The Momentus XT 750GB has impressively low noise levels at idle and under load. So does the Scorpio Black, which is only a fraction of a decibel louder. Props to Seagate for lowering the Momentus XT’s noise levels by several decibels; the old model is noticeably chatty in comparison.

Power consumption
We tested power consumption under load with IOMeter’s workstation access pattern chewing through 32 concurrent I/O requests. Idle power consumption was probed one minute after processing Windows 7’s idle tasks on an empty desktop.

The Momentus XT 750GB has to power two 7,200-RPM platters in addition to 8GB of flash memory, so I can understand why it consumes more juice than the Scorpio Black. Rather than tasking Adaptive Memory with speculatively caching data that would allow the drive’s platters to be spun down more often or for longer, data is cached only if it will improve performance.

Note that the SSDs consume close to the same amount of power as the hybrids and mechanical drives. Switching to a solid-state drive is unlikely to deliver a substantial improvement in notebook battery life.

The value perspective
Welcome to our famous value analysis, which adds capacity and pricing to the performance data we’ve explored over the preceding pages. We used Newegg prices to even the playing field for all the drives, and we didn’t take mail-in rebates into account when performing our calculations.

First, we’ll look at the all-important cost per gigabyte, which we’ve obtained using the amount of storage capacity accessible to users in Windows.

Even with the massive flooding in Thailand causing hard drive prices to skyrocket, SSDs remain an expensive proposition on a cost-per-gigabyte basis. The Momentus XT 750GB costs just 33 cents per gigabyte, while you’ll pay more than a dollar per gig for the cheapest of the solid-state options we tested.

Our remaining value calculations use a single performance score that we’ve derived by comparing how each drive stacks up against a common baseline provided by the Momentus 5400.4. This index uses a subset of our performance data described on this page of our last SSD round-up. Some of the drives were actually slower than our baseline in a couple of the included tests, so we’ve fudged the numbers a little to prevent those results from messing up the overall picture.

Our overall performance index rates the Momentus XT 750GB higher than not only the Scorpio Black, but also its desktop Caviar cousin. The new hybrid also scores higher than the old Momentus XT, which sits 25 percentage points adrift. As you can see, though, the SSDs are in another class entirely.

Now, for the real magic. We can plot this overall score on one axis and each drive’s cost per gigabyte on the other to create a scatter plot of performance per dollar per gigabyte. The plot gets a bit messy with all the SSD names included, so the solid-state drives are cloaked in gerbilesque anonymity. I’ve included the data points to illustrate the overall trend.

With better overall performance than its mechanical rivals, the Momentus XT 750GB sits atop the other inhabitants of the lower-left corner of our scatter plot. The drive’s relatively high asking price hurts its value proposition somewhat, but the premium doesn’t seem unjustified in light of the performance advantage the hybrid enjoys over the Scorpio Black. The SSDs deliver substantially better performance at a much higher cost per gigabyte, spreading the solid-state drives through the upper-right quadrant of the plot.

In general, there’s a bigger difference between the performance of SSDs and drives with mechanical underpinnings than there is between the individual members of each camp. That said, the plot clearly illustrates that performance tends to vary much more between different SSD models than it does with mechanical drives and hybrids.

Although this analysis is helpful when evaluating drives on their own, what happens when we consider their cost in the context of a complete system? To find out, we’ve divided our overall performance score by the sum of our test system’s components. Those parts total around $800, which also happens to be a reasonable price for a modern notebook.

With a suggested retail price of $245, the Momentus XT 750GB costs more than almost all of the 120-128GB SSDs we’ve tested. Solid-state drives offer a lot less storage capacity, of course, but this look at the numbers doesn’t take gigabytes into account. Although they might be in the minority among notebook users, some folks simply won’t need more than the 120GB of storage capacity provided by SSDs that cost less than the XT’s asking price.

Conclusions
The Momentus XT 750GB is a more refined hybrid solution than its 500GB predecessor. The flash memory still only works as a read cache, but it’s bigger, faster, and being used to greater effect. Seagate has upgraded the rest of the drive, too, giving it higher-density platters and a DRAM cache that’s just fast enough to take advantage of the 6Gbps SATA interface.

Our exhaustive suite of tests has exposed a couple of weaknesses in Seagate’s latest hybrid. The drive’s sequential transfer rates are a little behind those of the Scorpio Black 750GB in targeted benchmarks, and the XT’s poor performance in the copy component of our first batch of DriveBench workloads is cause for concern. Don’t be too concerned, though. The Momentus XT offers excellent real-world copy speeds, and it’s especially good at moving around large batches of small files.

While the Momentus XT boasts much better all-around performance than its Scorpio competition, Seagate must contend with a wave of solid-state drives that are faster across the board—in some cases by huge margins. The hybrid has one discrepancy on its side, though. With 750GB under the hood, the Momentus XT serves up hundreds of gigabytes more than the SSDs in its price range.

If you can squeeze everything you need into 120GB (or 250GB), then by all means spend a little bit less (or a little bit more) on solid-state storage. For everyone else, which probably includes most of the folks using notebooks as their primary PCs, the Momentus XT offers an excellent compromise between solid-state and mechanical storage. It would be nice if the drive were equipped with more flash and the ability to cache incoming writes, but the Momentus XT is still without peers. The only way to get anything comparable is to run dual drives with separate caching software, and that’s just not possible with a huge swath of notebooks.

I was completely sold on the Momentus XT 750GB when it was set to cost $189. With the drive now priced at $245, I’m not as enthusiastic. This is still a great alternative to standard mechanical disks for single-drive notebooks, and it’s TR Recommended for that segment of the market. However, this next-gen hybrid costs amost as much as a mechanical drive plus a small SSD, and I’d take the dual-drive combo in any system that could accommodate it.

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