I have a love-hate relationship with most SSD vendors. Why? Because I am a data hoarder, which means I’ve bought more SSDs than most people. But my drives also tend to last forever. And there’s a good reason for that: I never use up more than 80% of my SSD’s total storage.
On HDDs, you can get away with using 90–95% of the storage capacity as long as you’re fine with a performance hit. But filling up an SSD is one of the worst things you can do for its lifespan as well as its performance.
A full SSD has to work harder to save new files
There’s a lot going on behind the scenes
Modern SSDs store data in NAND flash chips, and the reason you shouldn’t fill your SSD past 80% boils down to how these NAND flash chips work. Each chip is divided into blocks, which are further subdivided into pages. A page is the smallest unit where data can be written, and is usually around 4 KB in size. While NAND flash storage is blazing fast, it has some quirks. The most important one is that it can’t overwrite existing data directly; previous data needs to be erased before new data can be written. Now this would still be okay, if not for the fact that NAND flash chips can’t erase individual pages; they need to erase the entire block. You can see how inefficient it would be to erase entire blocks each time data needs to be updated.
To mitigate this, your SSD controller maps each page to an address. When it receives a command to update the data on, say, address ABC, it will write the updated data to a new unused page and map address ABC to that page. All this while simultaneously invalidating the page which previously held address ABC. Over time, a block will accumulate a mix of valid and invalid pages. Since each NAND flash cell has limited program and erase (P/E) cycles, SSDs constantly move data around to prevent uneven wear. This process is called Wear Leveling.
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Most SSD slowdowns are just missed settings and neglected maintenance.
For simplicity, let’s assume your SSD contains a single NAND flash chip with two blocks, each containing eight pages. The first block has two valid and six invalid pages, and the second block has three valid pages and five unused (empty) pages. Now, suppose you have to copy six pages worth of data to the SSD. To achieve this, the SSD controller may copy the data from the two valid pages in the first block to two unused pages in the second block, so that the first block can be erased to accept new data.
In this example, writing six pages of data requires eight internal writes (six pages of new data + two pages of old data being written to empty pages). This extra work is known as write amplification. When your drive has less free space, the scope for wear leveling decreases, while write amplification increases, causing performance issues and extra wear.
SSD manufacturers already reserve space
But your 20% helps more than you think
To combat write amplification and premature wear, SSD vendors use something called over-provisioning: a certain percentage of the total storage capacity is reserved and inaccessible to you. This extra storage is used by the SSD controller for consolidating valid data and carrying out wear-leveling. Over-provisioning not only increases your SSD’s lifespan but also helps maintain consistent write speeds.
Wait, so if my SSD already has that extra space, why do I need to follow the 80% rule? Budget consumer SSDs typically have ~7–10% OP; premium/enterprise models offer 20–28%. It is usually enterprise-grade SSDs (or expensive consumer-grade ones) that see the highest amount of over-provisioning. So, that 20% is helping you more than you think, even with over-provisioning. In fact, Seagate’s blog claims that SSD performance begins decreasing once it crosses the 50% storage utilization mark. Your SSD will still last longer (and perform better) when it has that extra breathing room.
Don’t use SSDs for long-term storage; do this instead
They’re a terrible place to stash files you won’t touch for months.
Keeping your SSD around 80% is easier than you think
A few simple habits prevent slowdowns later
I don’t constantly monitor my SSD to ensure it stays strictly at the 80% level. It’s about being mindful of what you keep on your SSD drive and whether it even takes advantage of the SSD’s performance. In my case, I keep all my media files like videos, music, and pictures on my secondary hard drive since the SSD’s faster read/write speeds don’t offer a big advantage when accessing them. On the other hand, Windows and all my video games are installed on my SSD as the extra performance makes a discernible impact.
If you notice your SSD keeps filling up, it may be time to get a new drive or move some files to the cloud. Over a period of time, that extra investment is worth it, trust me.
