File copy to external SSD slows after three minutes

I have a Samsung SATA SSD (870 QVO 8TB) in an Orico 2139C3 drive enclosure (USB 3.1 Gen 1) connected to a USB 3.2 port on my desktop PC. When I initiate a large file copy from an internal SSD to the external drive, the copy runs at ~460MB/s for about the first three minutes, then abruptly slows to around 150-160MB/s for the remainder of the copy, for however long that takes. This behaviour is quite reproducible every time I perform such a copy.

Any ideas on why the copy always slows at this point, which corresponds to about 80GB having been copied.

Possible thermal throttling on either SSD with such a large continuous file.

Cache being full at this point?

I have noticed the same thing when copying large files to drives in my external dock. It's also 3.1 plugged into a 3.2 port. I even blamed the speed fluctuation on the dock and got another one. I found the answer someplace that said when the write buffer gets full the speed slows down. The jargon was over my head so I just accepted it and let it be.

You may need to manually initiate a TRIM operation since an external drive is often not left attached for very long and may simply need an optimization.

Try this:

In File Explorer, right-click the drive letter of the external SSD and select Properties. Go to the Tools tab and click Optimize. Select the external drive and click Optimize.

NOTE: The optimization may appear to finish very quickly, but this is the time where Windows provides "hints" to the SSD telling it what blocks should be trimmed. I would let it sit after that for a few minutes, then test again to see if it consistently performs better now.

EDIT: If that helps, let me know and I'll describe in detail in the morning what this is doing.

Try using FastCopy to copy large amounts of data. I know there is a bit of a learning curve, the documentation tends to be slightly confusing in a few ways, but it pays off nicely. IMO especially if you know how to use the command line options, it does.
Example 1 - Diff mode with Verify and a filelist:
Example 2 - Diff mode with Verify and specify the source directly instead of using a filelist:

To keep an eye on the temperature of the drive, use HWiNFO (portable version) with the option to show icons on the System Tray for Drive Temperature and Drive Temperature 2 (from the S.M.A.R.T. category) and edit the settings to adjust the polling interval of the S.M.A.R.T. data captures.

Here is a copy of my HWiNFO64.INI file:

You can make HWiNFO run automatically at startup. To get rid of the UAC prompt, add a task in Task Scheduler to make it run as an administrator (Run with highest privileges checkbox on the General tab of the task's Properties window) on every user logon (Triggers tab).

If you still want to be able to run HWiNFO like normal (i.e. without it automatically skipping to Sensors Only mode) you can keep a 2nd copy of HWiNFO Portable in a separate folder without putting the HWiNFO64.INI file in it (or still copy the file and edit the contents of the copied file) and create a separate shortcut if that's what you prefer.

Windows Defender and UAC scan files see disabling both will help.

The way an SSDs fast cache works can result in a huge performance drop when being fed large amounts of data, the reason being that when the cache is full the data needs writing out to the slower cells (this effect is much worse on QVL devices), if the amount of data going in is equal or exceeds the speed of writing then you will encounter a situation where everything has to slow down to allow data to be written out from the cache.
Normally the SSD waits for a lull in activity to write out the cache contents.

NickJP said:

I have a Samsung SATA SSD (870 QVO 8TB) in an Orico 2139C3 drive enclosure (USB 3.1 Gen 1) connected to a USB 3.2 port on my desktop PC. When I initiate a large file copy from an internal SSD to the external drive, the copy runs at ~460MB/s for about the first three minutes, then abruptly slows to around 150-160MB/s for the remainder of the copy, for however long that takes. This behaviour is quite reproducible every time I perform such a copy.

Any ideas on why the copy always slows at this point, which corresponds to about 80GB having been copied.

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This happens for everybody. It is just the crappy way windows handles file copies, coupled with ssd caches

If frequently transferring large files or folders with a lot of files it may help to adjust Virtual Memory to give Windows more room to work in. I always did Win95/98/98SE and WinNT 3.51/4/2000, later versions had more RAM and was not as big an issue. Of course, planning large copies to run at other times to not interfere with production helps.

Samsung QVO uses 4-bit cells, it´s a relatively low-cost, low-performance SSD. So the observed behavoir it to be expected, just live with it.

Subscriber43 said:

Cache being full at this point?

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At 80GB? What cache is that large?

bobkn said:

At 80GB? What cache is that large?

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A lot of slower SSDs use an amount of their cells in SLC mode as a cache to speed up overall performance, QLC drives do this and can have a cache size between 50 - 120 Gb, a percentage of total drive size.

DigitalGoat said:

A lot of slower SSDs use an amount of their cells in SLC mode as a cache to speed up overall performance, QLC drives do this and can have a cache size between 50 - 120 Gb, a percentage of total drive size.

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Thanks. I had no idea.

DigitalGoat said:

A lot of slower SSDs use an amount of their cells in SLC mode as a cache to speed up overall performance, QLC drives do this and can have a cache size between 50 - 120 Gb, a percentage of total drive size.

Click to expand...

Thanks, that's the explanation. I found a review of the very drive I'm writing to on Toms Hardware, and they tested this. See Samsung 870 QVO SATA SSD Review: Taking Baby Steps With QLC (Updated) - look for a para titled "Sustained Write Performance and Cache Recovery", where they have a series of graphs showing what happens to the write performance with sustained writes for 15 minutes.

NickJP said:

Thanks, that's the explanation. I found a review of the very drive I'm writing to on Toms Hardware, and they tested this. See Samsung 870 QVO SATA SSD Review: Taking Baby Steps With QLC (Updated) - look for a para titled "Sustained Write Performance and Cache Recovery", where they have a series of graphs showing what happens to the write performance with sustained writes for 15 minutes.

Click to expand...

That's what I said in the beginning.

With FastCopy, you should be able to see ~6.5% sustained write speed improvement until that 84GB cache is full and ~11% sustained write speed improvement until it no longer is full, i.e., when compared to the speeds that you have mentioned (and depending on factors).