How do Hybrid HDD/SSD work?

Discussion in 'Hardware Components and Aftermarket Upgrades' started by jhl1989, Dec 5, 2011.

  1. jhl1989

    jhl1989 Notebook Consultant

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    How do Hybrid drives work? From what I read (I'm gonna use the Seagate Momentus XT Hybrid Drive as the example) the Seagate Momentus XT Hybrid Drive is a 750GB HDD with 8GB SSD cache. SSD cache somehow recognizes your habits and boosts speed on certain things like opening files and so on. So is the 8GB cache accesible through "My Comouter"? Because I find it redundant that you do have a SSD in the hybrid drive but it doesn't speed EVERYTHING your PC does. If the 8GB cache isn't accessible why don't manufacturers make the "X" amount of GB's accessible? I think by making it accessible and usable wouldn't it work like a desktop PC with a SSD and HDD? :confused:
     
  2. hockeymass

    hockeymass that one guy

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    The way it speeds things up is by preloading frequently used files and programs into the flash storage. Obviously it can't preload everything in the drive onto the 8gb flash portion.
     
  3. jhl1989

    jhl1989 Notebook Consultant

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    couldn't they add a bigger SSD? Like a 128GB,256GB, or even a 80GB SSD would be good.
     
  4. Hungry Man

    Hungry Man Notebook Virtuoso

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    No, it does not prefetch based on frequently used files.

    It loads up file chunks based on predictions. This gives the illusion of memory because if you open the same thing multiple times it's already in the cache and it can make more accurate predictions. That is the extent of the memory.

    As soon as you open a separate file the cache is fully dedicated to that file/program.

    So basically, I hit "Chrome.exe" and it starts grabbing chunks that are related to Chrome.exe. That can mean drivers or whatever.

    This allows for the rather small 4-8GB to be very effective in speeding up the entire system as opposed to wasting it on a single application.

    Pretty much all caching works this way btw. It's just different algorithms for cache replacement/ management.

    They certainly could add a larger SSD. That would defeat the purpose though, which is to keep the drive cheap and fast.
     
  5. jclausius

    jclausius Notebook Deity

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    The Momentus XT v1 as well as v2 both use "Adaptive Memory" technology in order to improve "READ" speeds for the drive.

    Here are the nuts and bolts. ALL data is stored on the platters. You only see one main drive (500GB or 750GB). The SSD NAND flash portion is invisible and is only available internally to the drive's controller. The OS/User cannot independently write to the SSD flash, as it's purpose is to serve up blocks of data thus acting as a very fast READ-only, persistent data cache. So only frequently blocks of data on disk that do not change eventually make it into the SSD flash portion - available for any READ operation when requested. This is all handled by the XT's "Adaptive Memory" caching technology.

    The XT's Adaptive Memory cache routines target frequently accessed data at the disk's block level rather than the file level, so the SSD portion is OS and filesystem independent. As more and more data blocks are requested, the drive "learns" what block addresses are being accessed over and over again, and the XT will then copy these requested blocks of data into the flash and maintain the integrity of the data b/w the SSD portion and what is stored on disk.

    Also, there are some cool NEW things about the XT v2 including a feature of FAST Factor which prevents blocks storing boot-up information from being invalidated from the SSD. This means boot-ups on the XT v2 will keep all data holding blocks accessed during boot-up in READ cache, where the v1 disk may remove that data from the cache over long stretches of computer use with heavy data reads.
     
  6. funky monk

    funky monk Notebook Deity

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    How does partitioning a hybrid drive work? Could you for example set it up so one partition had sole access to the cache, whereas the other was a simple partition?
     
  7. jhl1989

    jhl1989 Notebook Consultant

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    So it only boosts read speeds? And it doesn't boost write speed when let's say moving a a large file to the HDD?
     
  8. Phil

    Phil Retired

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    Only read speeds.
     
  9. kent1146

    kent1146 Notebook Virtuoso

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    The drive looks like a regular 500GB / 750GB drive to the system. No special configuration, partitioning, or setup needed. It is used and accessible only by the drive itself, for caching purposes.

    Think of it this way... a regular hard drive today comes with a small cache (8MB, 16MB, or 32MB). There isn't a way for you to directly access that cache. It's just there for the drive to use, based on its own internal algorithm for caching data.

    A Hybrid SSD is exactly the same concept. Except instead of a small 8MB / 16MB / 32MB cache, they chose to use a 4GB cache based off of the same kind of memory chips used in SSDs (NAND flash memory).
     
  10. jclausius

    jclausius Notebook Deity

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    No. All blocks in the drive are subject to the Adaptive Memory caching routine.

    Now in the XT v2, the FAST Factor for boot stuff looks rather interesting in that it probably looks at the MBR, and follows that chain of drive reads needed to boot a system. That data is stored differently in that it cannot be invalidated by reads from disk AFTER the system has booted.
     
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