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I would like to know how does Seagate Momentus XT see virtual disk?

  1. As a single huge file (size depending on virtual disk type)
  2. As individual file within this virtual disk?

If the first one is true, then virtual machines on this drive work just as slow as if they'd be on everyday platters disk.

If the second one is true, hybrid drive speeds up virtual machines as well. You would likely see bootup times much shorter on a hybrid drive.

So users (or people that know its internals), which one is true?

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3 Answers 3

up vote 1 down vote accepted

The harddrive controller sits lower than the OS, so I would expect it to cache sectors and not necessarily certain files.

This makes more sense considering one of the biggest areas where the Momentus XT has shined is boot times.

But since the caching implementation isn't documented, you'd have to benchmark to verify this.

Edit:

This makes even more sense as Seagate claims the algorithm is OS independent.

Also here: http://forums.seagate.com/t5/Momentus-XT-Momentus-Momentus/Momentus-XT-Frequently-Asked-Questions/td-p/52701

The Momentus XT Adaptive Memory algorithm is an LBA-based algorithm that looks for small portions of data that take the drive a disproportionate amount of time to access. It then puts these portions of data in the Solid State storage for quicker access

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As it turns out Seagate said they cache LBAs... –  Robert Koritnik Feb 9 '12 at 17:21

XT hybrids work just like a platter drive, because they have platters, they also have a large amount of cache ram, the memory controller on the hard drive caches the most used/accessed files into the ram for fast loading into the motherboard memory. Virtual disc images are single files and are so large they may never get cached into the small hybrid ram, so there is no advantage.

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I suppose that @surfasb provided a more accurate answer. And I also searched the web and it actually seems that virtual machines also benefit when stored on a hybrid drive. –  Robert Koritnik Feb 9 '12 at 9:06
    
"seems" please post your research links. I don't believe it makes large vm's run any better, and willing to bet no one has done any valid study of it, only "vm's seem to run better on a hybrid" opinion. There is just not enough memory on a hybrid drive to keep a vm cached all the time, its all speculation since one cannot access the cache on a hybrid drive to monitor what is cached and what is not. All the Seagate Hybrid literature says it caches "files" not sectors of data, so your choice of best answer is based on what "seems" correct to you, utter nonsense. –  Moab Feb 9 '12 at 16:42
    
It's not about caching whole VMs... It's about caching most accessed LBAs that don't change (too often)... Most of OS files are only read. And this is true for desktop as for VMs as well. I see no reason why VMs wouldn't benefit from a hybrid drive. Actually they can benefit just as much as any desktop system. The only issue I see is if you're frequently running several VMs at once, because SSD part mostly gets filled with data OS data that's repeated several times. Same data, multiple times. –  Robert Koritnik Feb 9 '12 at 16:54
    
And talking about links... I don't have to post any links because as it turn out Seagate states themselves they're saving most frequently accessed LBAs and not files. So that may be whole files or parts of them. Doesn't really matter whether they're actual files or files in a VM. From a disk controller point of view both are just data stored in blocks. LBAs are those that are cached. –  Robert Koritnik Feb 9 '12 at 16:56
    
"they're saving most frequently accessed LBAs and not files" where, substantiate your claim. All documents I read said otherwise....seagate.com/www/en-us/products/laptops/laptop-hdd –  Moab Feb 9 '12 at 20:20

The SSHD only caches LBAs not files. It connects via the SATA cable and so the SSHD has no knowledge of the OS or its file structure, it just sees read/write requests for data blocks or LBAs. Notice Seagate says "DATA" not "FILES"

From Seagate: "Seagate Adaptive Memory™ technology effectively identifies the most frequently-used data in your computer. This data is stored in the ultra-fast NAND flash memory which results in blazing fast boot times, and ... "

http://www.seagate.com/internal-hard-drives/laptop-hard-drives/laptop-solid-state-hybrid-drive/

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