SSD write speed drops at about 82-84% of the way into a file transfer

Question

I am running into an issue where both my SSDs drop to about 1-2% of their write speed. This usually occurs when the file I'm transferring in-between them is about 84% complete.

I have two different type of SSDs: The one I have installed Windows on is a Western Digital Blue 3D NAND SSD 250GB (WDC WDS250G2B0A-00SM50), and the other one is a Kingston SSDNow UV400 480GB (Kingston SUV400S37/480G). These are supposedly to have 550MB/s read and 500-525MB/s write.

At first, when the transfer is initialised, the numbers skyrockets, and they keep on going up till they hit 82-84%, and then they plummet to between 2-14MB/s (keep in mind that this is between the two SSDs).

I have tested with a folder of multiple files of about 8GBs, and a single large .mp4 file of about 12GBs, still same results .

I have tried just about every solution I came across, like; Enabling and diabling AHCI, enabling and disabling TRIM, updating BIOS, using RamCache II to allocate more than 20GB of RAM to both SSD, but still no improvement at all. The AMP is enabled and set to maximum. These are just about new SSDs; the WD one is around a year old, whilst the Kingston one is just over two years old. The WD SSD is about 25% full, and the Kingston one is just over 5% full.

What should I do to resolve this issue?

Answer 1

I suspected that the slowdown might be due to SLC cache saturation, and it seems very likely to be the case after a bit of research.

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Simply put, just because the drive advertises 500MB/s, does not mean that you will always reach 500MB/s. Lower-end drives often reach this speed momentarily using some cache strategy. This is usually fine, because most day-to-day use does not involve transferring large files. However, as soon as the cache is saturated, the drive will revert back to a much slower storage medium.

WD drive seems to be clear, since its 3D TLC does not rely on SLC mode cache for burst speed. But the Kingston drive does seem to use this strategy. Therefore it's likely that the Kingston drive filled up its SLC cache quickly, then dropped back to writing to its 2D TLC, which is in fact not much quicker than HDDs. See here on the sequential steady state write performance of Kingston UV400.

On top of this, there is also the overhead introduced by NTFS; there could also be other system bottlenecks in play. 1-2% of full speed is a bit extreme if you are indeed transferring large files (>50MB), but it's not too surprising either.

In terms of what you can do, there is honestly not much. It's just a common pitfall of cheap SSDs that you have to deal with. This is a part of the reason you will see enterprise-grade drives that advertise similar speeds superficially, but at several times the price. If you look into their detailed review, you will find that they often have very consistent steady state random read/write performance, sometimes a dozen times higher than commercial-grade drives.

I would suggest researching the SSD you are looking to purchase beforehand - specifically reading reviews from reputable sources such as Tom's Hardware and The SSD Review.