What is Hard Drive and SSD Cache, and What Does It Do?

PCs are complicated machines, filled with dozens of smaller components, all working together. Anyone who’s worked with PC hardware is familiar with the main drive specs, like capacity, read/write speeds, and platter rotation speeds for hard disk drives. However, there is a lesser-known and often overlooked feature that impacts the speed and performance of your storage drives. The feature is known as random access memory (RAM) or cache for hard disk drives (HDDs) and dynamic random access memory for solid-state drives (SSDs). Let’s take a quick look at what the HDD and SSD cache are and how they work.

What Is Hard Drive Cache?

The hard drive cache is often known as the disk buffer. By that name, its purpose becomes a little more transparent. It acts as a temporary memory space while the hard drive reads and writes data to the permanent storage on the platters.

You can think of a hard drive’s cache as RAM designed explicitly for the hard drive. Hard drives have built-in microcontrollers that govern and process data coming in and out, much like a CPU. The HDD RAM works alongside the microcontroller to store memory as it’s being processed.

You can also think of hard drive cache as something similar to buffering regarding streaming content. Everyone has dealt with streaming a video on a slow connection. The video player waits before or during playback to collect data to continue playing the video more smoothly as it progresses. The hard drive cache allows a hard drive to do the same when reading and writing data.

How Does HDD Cache Work?

As the HDD reads and writes data, it pulls it from the platters. Very often, the HDD is working with the same data repeatedly since the person using the computer is usually working on one or two tasks at a time. The HDD holds data in its cache that you or your programs are using most frequently and, most recently, which eliminates the need to pull it from the platters each time that the data is needed. This action speeds up the drive’s performance.

HDDs: Reading Ahead and Behind

Typically, a hard drive doesn’t just pick up the data it needs. It also reads the data around it. Hard drives aren’t efficient. The spinning platters and read/write heads are inherently limited by physical moving parts, which are much slower than SSDs with no moving components. Therefore, hard drives try to compensate by guessing.

When a user or a program requests data (reminds me of Tron), the hard drive reads that data and the data around it from the platter and stores it all in the buffer. Since there is a decent likelihood that the surrounding data is similar, the drive assumes that the user or process will also request the surrounding data soon.

HDDs: Evening Data Flow

There are many different steps to retrieving data from a hard drive. Each one of them takes time, and they rarely sync up. Transferring data from the hard drive via SATA usually moves faster than the HDD can read and write data to the platters. The disk buffer is often used to even out this flow of data and make the process much smoother.

HDDs: Minimizing Wait Times When Writing

Again, hard drives are slow. They are probably the most time-consuming part of any computer because of their physically moving parts. Writing data is usually “painful” to the user.

HDDs: The Purpose of Cache

HDD cache (RAM) helps to speed up the data-writing process by virtually fooling the rest of the computer. A hard drive will take data into its cache and begin writing it. Instead of waiting to write all the data onto the platters, the HDD signals the computer that it did. The PC or Mac either continues sending more data or moves on to other tasks, believing that the process is complete. Either way, this allows the computer to continue to the next event.

However, there is a downside to caching data. While the hard drive tries to make good on its promise to write the data, it can lose it. If the computer is powered off suddenly, all the data stored in the cache will disappear. RAM/cache is volatile storage.

Speeding Up HDDs

Higher HDD RAM (cache) doesn’t mean faster drive performance on single tasks directly. It’s not like it’s causing the drive to move faster. Having a disk buffer does, however, allow a disk drive to multitask much more efficiently, and chances are, that’s something you need.

It’s rare that a drive does just one thing or will only interact with one process at a time. Disk-based hard drives are still well-known storage devices in modern PCs. However, SSDs are gradually replacing those HDDs. Even with a single task, multiple programs may need to access the drive’s storage simultaneously. You may be working with two or more files, experiencing background tasks, or even receiving updates.

Servers are another space where having some RAM in hard drives is critical. Server HDDs are always going to be doing multiple things. Think of a database behind a website. Whenever a user completes an action that the website has to store or log, the site accesses the info and writes it to the database. Every time someone even views that website, it reads from the database. It would be rare that the drives storing that database wouldn’t be performing multiple tasks simultaneously.

What is the Cache In SSDs?

SSDs aren’t as slow as physical hard drives, so do they need some cache, too? In short, they do. While RAM in hard drives behaves like RAM in the PC or graphical processing unit (GPU), the cache in solid-state drives serves as DRAM. It’s much faster and keeps pace with the SSDs.

Even though SSDs are much faster than their disk-based counterparts, cache still delivers benefits. Solid-state drives still use it to regulate input/output and provide faster read and write access. Meanwhile, some SSDs don’t have built-in DRAM. It saves on power consumption but forces the drives to compensate in other ways.

Another thing to note is that SSDs write in batches rather than sectors, so they have to load groups of data, add to it from the cache, and then put it back and move on to the next group of data. That is why the DRAM cache is beneficial.

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Overall, the cache does matter, even in SSDs. It isn’t as important as the primary drive specs, but you should consider it. If your HDD is multitasking or running continuously, like with a server, for development, or even for gaming, look for larger RAM sizes. You’re going to see the most benefits from it. Home users looking for a storage drive for occasional use don’t have to worry about it as much, although several tabs in a browser alongside a YouTube video and two spreadsheets while also receiving updates can slow things down. For SSDs, the waters are a little murkier, but it’s still worth considering DRAM in your decision process. Other factors can easily overshadow it, though.