Articles: Storage

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Mixing together two technologies which are developing in parallel may often produce the compromise that users are looking for. When it comes to hard disk drives, we can recall Western Digital’s Raptor as a product that combined a spindle speed of 10,000 RPM typical of server HDDs and a SATA interface available on ordinary desktop PCs. The resulting super-fast desktop HDD enjoyed high demand.

Today, we can also see two technologies developing in parallel: traditional HDDs offer huge storage capacities at low prices whereas solid state drives deliver high performance but have a high cost of storage per gigabyte. Could there be a compromise? The simplest way is to take one HDD and one SDD and use them together, each for storing certain types of data. But what do you do if your notebook has only one disk bay or if you just don’t know beforehand what data will require quick access? The solution is offered by Seagate in the way of the hybrid Momentus XT drive. We will compare it with ordinary HDDs to see what benefits it brings us.

Testing Participants

Seagate Momentus XT: ST35005620AS, 500 GB


The new series is no different from ordinary 2.5-inch drives externally. The continuity is also emphasized by the use of the Momentus brand. The XT suffix must have been borrowed from the desktop Barracuda XT series. The HDD part of this hybrid device is interesting in itself, even though not exceptional. It is a 2.5-inch HDD with one or two 250GB platters rotating at 7200 RPM. It has 32 megabytes of cache memory (the previous Momentus 7200.4 series used to have only 16 GB). We’ve got the highest-capacity (500 GB) model for our tests.

The SSD part of the Momentus XT consists of 4 gigabytes of SLC flash memory. You can see the chip in the bottom right of the photo: it is a Micron 0FB12 MW167.

What can this flash memory do? You don’t have to read Seagate’s presentations to understand that if you know just some basic facts. What is the main advantage of flash memory over HDDs? Yes, it has a lower read access time. It’s hard to improve this parameter with HDDs because it is determined by such slow (in comparison with the speed of an electronic chip) operations as moving the read/write head to the necessary track and rotating the platter, but it is often the case that the user accesses the same and rather small amount of data rather than all data stored on the HDD. So, if such data is cached in flash memory, it can be retrieved very quickly during the next access, without waiting those 12-15 milliseconds for the head to meet the necessary sector.

What else can flash memory do in an HDD? Theoretically, it can be used to cache write requests but 32 megabytes of traditional cache installed in the Momentus XT are going to do that just fine. This cache memory doesn’t cost much and write caching doesn’t require too much memory, anyway. The flash part of the HDD can also store some auxiliary information which is being regularly updated, but that’s a trifle. The biggest effect is going to be produced by caching read requests. But as noted above, it will only work when the user is accessing the same piece of data over and over again. Many benchmarks may not even notice it. Well, that only makes our test session the more interesting.

As for opponents, we took two 500GB/7200RPM drives from our latest comparative review of 2.5-inch HDDs: a Seagate Momentus 7200.4 and a Hitachi Travelstar 7K500.

And there are also a couple of HDDs we have not tested before.

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