A Solid State Hybrid Drive (SSHD) is basically a hard disk with NAND to cache frequently accessed data. To the operating system, it accesses it exactly the same as a regular hard disk, so there is no need to install any driver or software to use one. The SSHD itself decides what to put in its NAND cache by monitoring which sectors are frequently accessed. Some SSHDs even cache written data to speed up write operations and then later write it to the spinning platters when the drive is not busy.
One of the main concerns people have with SSHDs is that its NAND is too small to be of practical use for an operating system, particularly when numerous applications are installed. For example, the Seagate Laptop Thin SSHD we demonstrate has a 500GB capacity, but just 8GB of NAND. Sure enough, there are plenty of videos on YouTube showing a clean Windows installation launching lightning fast from an SSHD, so some may be wondering what happens over time as a large number of applications are installed and regular upkeep optimisation is neglected, such as where a traditional hard disk takes several minutes just to reach the desktop.
Well, in this video, we have a Windows OS that takes over 4 minutes to boot to having the web browser open at the homepage with a regular 3.5” 7200RPM 320GB hard disk. The OS was already on an older Seagate Momentus XT 250GB (+4GB SSD) hard disk, so a 1:1 mirror image was made to the hard disk and to the new Seagate Laptop Thin SSD. Each drive was booted followed by launching 5 applications (IE10, Firefox, Gimp, Word and Windows Media Player) five times in a row to give each SSHD a chance to learn what to cache and also for the HDD since Windows 7 does its own start-up optimisation.
As this video shows, it’s pretty clear that the SSHD does a major transformation to the boot and application launch performance, without doing a single tweak. Unlike frequently trying to improve the start-up performance by removing software from the Windows start-up, defragmenting, cleaning out temporary files and so on, there is no worry about the OS slowing down again over the coming weeks as new start-up processes start getting added back in, temporary files build up again and so on.
So why do SSHDs have such a small NAND capacity? In reality, even when the drive is mostly filled, only a tiny portion of that data is frequently read, such as the files loaded during boot, launching applications and directory indexes which are read when searching for files. Even though the OS itself may be well over 20GB, the vast majority of its files are rarely read. Examples include most bundled windows applications (e.g. Windows Easy Transfer), help files and the vast number of DLL & shared resource files that few or no currently installed applications use. The same goes with most applications that the user installs over time. To see a real world example, just open the program listing in the start menu and count how many items that were not used in the past week.
The sectors these rarely accessed files use will not be stored in the cache, but should a time come when the user installs or uses something that puts certain files into frequent use, the SSHD will automatically move them into the cache. Files that are later on less frequently accessed are in turn removed from the cache.
To show how effective an SSHD decides what to cache over a longer period of time, the bottom-right screen in the video shows what happened when we reinitialised the older Momentus XT drive and ran it through its relearning process again. It performed exactly the same as before for booting to the desktop and just slightly quicker when launching the 5 applications. What this shows is that even though the older Momentus XT SSHD was used for nearly a year, the SSHD did a very good job at making effective use of its tiny 4GB cache to keep the boot time and application launch times fairly consistent.