Hardware

TPU does a test where they record the speed while filling up the whole drive, should give you a good idea: https://www.techpowerup.com/review/corsair-mp600-mini-1-tb/6.html

The slowdown is write speed, if you read from the drive mostly, don't worry about it much.

Tom’s Hardware usually does longer write tests with iometer to analyze cache size and performance however I think it may only be a 15 minute run. Here’s their review of the 2tb samsung 990

However it’s becoming less of an issue with modern ssd’s that incorporate hybrid caches and faster NAND. As long as the drive has a large enough static cache to absorb most of what you throw at it or fast enough NAND to at least be bearably fast it’s not that big of a deal. You just have to know the NAND’s limitations. Even drives like the crucial p3 are fine as long as you understand eventually writes will slow down to 80MB/s or so.

For secondary storage it’s less important but I would still try to leave some free space on OS drive for fast swap and temp file performance

Out of all the gen4 SSDs, SN850x is the least affected by performance degradation.

https://old.reddit.com/r/hardware/comments/1146b0s/ssd_sequential_write_slowdowns/

I don't keep up with SSD benchmarks, but the mechanism behind this phenomenon is not anything mysterious. Most consumer SSDs ship with TLC or QLC NAND, which supports either 3 or 4 bits of storage per cell. However, writing the full 3 or 4 bits is slower than just writing one bit in each cell, so the drives use available empty NAND as an SLC write cache while the drive is still not full. So when you write to a relatively empty drive, your data will go into the DRAM cache on the controller (if there is any being used as a write buffer) and then get written into available NAND chips in SLC mode. Later on, the drive will consolidate the data down into QLC/TLC properly, but you get the advantage of a fast write so long as there is enough empty NAND to use SLC caching.

Obviously this falls apart once your drive gets close to full and there are no available empty NAND chips to write to as SLC cache. This is also why the write performance of budget drives tends to drop off worse than higher end drives. The nicer drives have faster NAND and usually have DRAM on the SSD controller to help performance in the worst case. Enterprise drives often sidestep this issue entirely via just using SLC or MLC NAND directly, or by having additional overprovisioning (extra NAND chips).

I just overprovision my SSD's. Sata and Nvme.

I think storage review (on enterprise products) preconditions the drives for a while before testing, and they might be kinda full for testing?

Serve the home also tests against a moderately full SSD. But that's their only set of data.

I just go with /r/NewMaxx ‘s advice

She usually has a graph that shows this.

https://youtube.com/@TechTesters

Almost everything you hear is exaggerated. Fill the drive. Leave some room to work with is all.

You can always buy enterprise grade disks if you are concerned about this.

Effect it's still present, but with more spare flash becomes less pronounced

It's also bad for durability as it impairs wear leveling

This is precisely why many people still suggest hdds for stuff that does not need to be as fast.

Does anyone test playing games for long periods of time on external SSD's?

If you don't do much writing or mixed IO (e.g., things like a database), it doesn't really matter how full you want to keep your SSD. Writing slows (and write amplification goes up) as the disk fills because garbage collection has to work harder. As you may well know, NAND media consists of a set of blocks, and each block contains a set of pages. Writes are at the page level, but erases are at the block level. As data is overwritten or trimmed, some data in a given block will no longer be valid. When a block is garbage collected, the penalty lies in the amount of still-valid data that needs to be copied to another block. The fuller you keep the SSD, the more laden each block is with data you still care about, which has to be moved each time garbage collection is invoked. But if you're doing read-mostly work, it probably doesn't matter.

When you overwrite data on your flash disk it doesn't just go to the same location and replace the data like a hard disk, it writes to a new location and saves that location as "valid" for a given block address.

NAND cells are erased when you need to free up space for new writes - but the erase block is large, so much so it's likely that what you're erasing contains data you need to save. You have to move that valid data to a new place and erase the block and this slows the whole operation down.

If you have very little unallocated space on your disk - more and more operations will require this shuffle and this shuffle is less efficient (you can't wait to erase an 'optimal' block because they all contain 90% or more valid data that must be moved). A packed drive is therefore less efficient, but 2 different size drives at the same % fill won't see the same performance loss because the larger one still has more scratch space to work with (most consumer drives set this aside as an SLC 'cache' to enable snappy performance and a more efficient stripe packing, but that cache shrinks as the drive fills.)

Tl;Dr - you don't need to save 800GB of your 4TB drive, but saving 200GB or so will keep you from seeing any performance loss. If you use any consumer SSD in a 24/7 server workload they'll all hit a wall eventually because they're not designed for sustained performance.