Understanding SSDs

Why solid-state drive (SSD) performance slows down as it becomes full

Have you ever wondered why your solid-state drive degrades performance over time? Here we look into the problem and what you should do to keep the drive at top speeds.

Solid-state drive laptop upgrade

If your computer has a solid-state drive (SSD), you probably noticed that as you fill it up performance slows down dramatically. You’ll see this when opening apps and copying files will take longer than usual, and sometimes this will also cause freezing issues making your device harder to use.

Now you might probably be asking: Didn’t SSDs supposed to be faster? The answer is yes, solid-state drives are faster than traditional rotating platter hard drives. However, there is a performance problem due to the way these new drives technology and NAND Flash storage work.

SSD blocks and TRIM

Solid-state drives work differently than traditional hard drives, though they accomplish the same thing — they both store data.

When you’re working on a file, and you’re ready to save it, the solid-state drive finds the necessary empty blocks and writes on them to store your data. The process of filling an empty block with data is the fastest way to write to the storage, which is the main reason operating systems, such as Windows 10, includes support for TRIM.

TRIM is a feature used by an operating system to command the SSD to find and delete any data on blocks that make up a file you deleted, so that later, the drive can quickly fill them up again with new data.

If the blocks are not empty, then the solid-state drive must delete the data inside them before information can be written again, which can add significant delay affecting performance.

As you continue to store data and storage fills up, the drive will begin to run out of blocks, and because solid-state drives can only write data in pages of 4KB-8KB inside of a 256KB block, you end with blocks that are not completely filled.

NAND flash memory example by Dmitry Nosachev

The problem is that SSDs can’t use the leftover space in a block to write new data directly, because it would actually destroy any data already on it.

The solution to this problem is a write operation that loads the data inside of a block into cache, modifies its content adding the new data, and then write pages back to the block.

The only caveat is that the entire process takes time, and repeating the process on many blocks reduces speeds dramatically, and therefore affecting performance.

Overcoming SSDs performance issue

In order to prevent solid-state drives from degrading performance, many drive makers add extra storage to each drive that isn’t available to the user, but it helps to ensure SSDs don’t get completely full keeping speeds in a more acceptable level.

In other words, adding extra storage reduces the need of having to modify a partially filled block to add new data. The solid-state drive will end up with more partially filled blocks, but it ensures performance won’t decrease quickly.

In addition, solid-state drives include a “garbage collection” (GC) solution that also works to overcome the performance issue. Once the storage reach capacity, the garbage collection will look for blocks that aren’t completely filled and it’ll consolidate them to free up space.

Keeping SSDs speedy

While solid-state drives are ridiculous faster than traditional rotating platter hard drives, they dramatically slow down performance as you fill them up.

The rule of thumb to keep SSDs at top speeds is to never completely fill them up. To avoid performance issues, you should never use more than 70% of its total capacity.

When you’re getting close to the 70% threshold, you should consider upgrading your computer’s SSD with a larger drive.

If you’re not up to the challenge, and you need more storage, you have a number of alternatives.

You could get an external hard drive that you can connect using a USB cable. For example, a good choice is the Seagate Expansion 8TB external hard drive, which adds a ton of additional storage to store movies, music, photos, and documents for a reasonable price.

Seagate 8TB external hard drive (STEB8000100)
Seagate 8TB external hard drive (STEB8000100)

If you need more storage, but you also need SSD speeds, then another good choice is the Samsung 850 EVO 500GB internal solid-state drive.


Samsung 850 EVO SSD
Samsung 850 EVO SSD (MZ-75E500B/AM)

Then get an inexpensive external 2.5-inch hard drive enclosure like the one from Inateck, mount the solid-state drive, and connected to your computer.

Inateck USB 3.0 external hard drive enclosure (FE2004)
Inateck USB 3.0 external hard drive enclosure (FE2004)

Keep in mind that solid-state drives performance is not only affected by the amount of data you store in them, but also performance will vary per manufacturer, components quality, features, and grade level. 

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