Solid-state drives are much faster than HDD drives. While regular Disk drives may be slow, but they offer a very large storage capacity at a very low price per gigabyte.

To get the advantages of both, Some user do mix these two type of drives in system, SSD for application, while regular large HDD for backup, background running etc..

There are also some ways to utilitize SSD cache at block device layer on top of HDD, such as LVM cache logical volume, bcache etc..

Is there a way to combine them together? Hybrid Drive is one of linux cache options.

 

Hybrid Drive is a logical or physical storage device that combines a fast storage medium such as NAND flash solid-state drive (SSD) with a hard disk drive (HDD), with the intent of adding some of the speed of flash storage to the cost-effective storage capacity of traditional HDDs. The purpose of the SSD in a hybrid drive is to act as a cache for the data stored on the HDD, improving the overall performance by keeping copies of the most frequently used data on the faster SSD.

 

Hybrid hard drives promise some of the performance of a solid-state drive with the capacity of a mechanical drive. They’re bigger than an SSD and faster than a plain-old mechanical drive.

These are sometimes called “SSHDs” — solid-state hybrid drives. The drive automatically caches data in the solid-state storage for you, offering faster speeds for the files you use most.

Hybrids Are Regular Hard Drives With SSD Storage

A hybrid hard drive contains both a traditional magnetic drive and the amount of solid-state storage you’d find in a small solid-state drive. Importantly, this hard drive appears as a single drive to your operating system. You’re not in charge of deciding which files go on the mechanical drive and which files go on the solid-state drive. Instead, the drive’s firmware manages what is and isn’t on the solid-state drive.

The SSD portion of the drive acts as a “cache” — files you access frequently, such as your operating system files and program files — are stored on the SSD portion of your drive by your firmware. Although this is a cache, it’s stored in non-volatile solid-state memory — that means it persists across reboots, so it speeds up your startup process.

The goal is to have the drive access system and program files with the speed of a solid-state dive, and provide the storage capacity of a magnetic drive for other files. The drive handles this on its own — you don’t have to shuffle files around or decide what goes where.

How does it work

In contrast to other Linux cache options, the host doesn’t have direct control over data in a hybrid’s flash resources. Hybrid drives carry their own storage controller within the drive. The controller—and specifically the
algorithms running within it—dictates what data should reside in the cache and whether incoming read requests should seek from the NAND or magnetic media.
More importantly, data transfer between the flash media and platters is done within the hybrid drive and controlled via onboard ASICs. With only one component in play, hybrid solutions can cost less. There’s no need to redesign systems to accommodate additional flash modules.

Furthermore, if users need to upgrade the storage subsystem, they can simply replace the HDD and leave the mSATA SSD in place decreasing upgrade cost.

How much NAND SSD is needed in Hybrid Drives?

How much NAND is needed for storage acceleration? Seagate researchers have determined that the average office worker needs considerably less than 8GB of flash to realize the majority of storage acceleration benefits. More NAND may increase endurance, but the average user is unlikely to gain more performance for the added cost. Even with just 8GB, users are unlikely to meet NAND endurance thresholds within 10 years.

How is the performance

I did not check other Vendors product specs, just picked Seagate 15k HDD with TurboBoost enhanced cache models(HyBird Drive)

Seagate Enterprise Performance 15K HDDs kick performance into a higher gear with TurboBoost-enhanced cache models:

  • Accelerates I/O operations and optimizes response times (up to 20× over base drive models) completing more transactions faster, even during peak demand
  • Uses traditional NAND and advanced algorithms to promote hot data and achieve performance improvement (up to 3× over base drive models) in real life workloads
  • Delivers more predictable performance while protecting data from corruption due to unexpected power loss

Conclusion

Hybrid drives is cheaper than solid-state drives because they contain a smaller amount of solid-state memory, but expansive than HDD. As flash cache and SSHD options expand, the price gap against conventional HDDs will likely continue to narrow. Compared to paying for more RAM or a faster CPU, having NAND flash integrated into the HDD subsystem cost-effectively delivers the necessary acceleration.

A hybrid drive is also a single physical drive, which can be a big advantage. If you want both solid-state speeds and a large amount of storage space, having a hybrid drive may be simpler solution.

At some point—and we may be there in the mobile space—users will struggle to justify why they should buy a traditional hard drive with no flash acceleration.