The fluctuating write speed of USB drives, characterized by a “wave-like” pattern, is caused by the data relocation process in the USB’s solid-state storage management algorithm. This is a common phenomenon in the field of solid-state storage, with USB drives showing it more prominently (mainly because USB drives have lower performance requirements). In contrast, products like eMMC and SSDs handle this more effectively, so the phenomenon is less noticeable.
To explain why these fluctuations occur, let’s first cover some relevant concepts. A USB drive consists of a controller chip and several NAND Flash memory chips. The data management in NAND Flash follows a Page and Block model, with the following key characteristics:
- A Block consists of multiple Pages.
- Data is written and read in Pages, each typically containing 32 sectors.
- Pages can only be written in sequence, not out of order.
- Data erasure must be done at the Block level.
Now, let’s discuss why the large speed fluctuations (the “waves”) happen. Most of the NAND Flash used today is TLC (Triple-Level Cell) memory, and TLC, on top of the characteristics mentioned above, has an additional challenge—it requires repetitive, iterative operations to fully write data to a Block.
In fact, to simplify the management algorithm, TLC writes data in entire Blocks. Therefore, many USB drives today use this type of management approach.
When a USB drive receives data from the computer, it initially stores the data in SLC mode (Single-Level Cell), which doesn’t have many restrictions and can flexibly handle various operations from the computer.
Once a certain amount of data is stored, the controller selects three SLC Blocks and performs a CopyBack operation, which transfers the data from the SLC Blocks into a TLC Block according to TLC’s writing rules. This CopyBack operation is time-consuming and significantly reduces the write speed during this process.
Here’s a visual representation to understand the speed difference:
The time taken for a single CopyBack operation is roughly 10 times that of writing to an SLC Block.
So, the fluctuating speed of a USB drive during file transfers—sometimes fast, sometimes slow—isn’t related to the type of data. Once a certain data threshold is reached, triggering a CopyBack operation causes the speed to drop dramatically. Of course, smaller files can also slow down the speed, but that’s a different issue.
As technology advances, there’s increasing attention to improving the smoothness of USB drive speeds, and some high-end manufacturers have developed USB drives with more consistent speeds (although they may be slow without much fluctuation).
Additionally, because USB drives do not support TRIM (a command that optimizes space in solid-state storage), the speed will inevitably degrade over time. Many users might think that deleting files or formatting the drive clears the data, but that’s not the case. The SCSI protocol used by USB drives does not have a delete command. When files are “deleted,” they are merely overwritten with zeros or marked as deleted in the file system’s header.
Formatting the drive only rewrites the file system, making the files untraceable, but the data is still present. The only way to truly clear the drive and restore it to its original performance is to remanufacture (reflash) the USB drive.
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