Status during USB file transfers

[ALF__]

So, i have this strange problem that ive never encountered before.

When i mount a USB-stick, and transfer files to it, the "status" is finished almost instantly. But the transfer is still going in the background.
This is systemwide, so its not only true for a specific application.

For example, 400mb takes a while, but the status says it is finished in about 2 seconds. However, the LED on the stick keeps blinking for the right amount of time.
It is also noticable because when unmounting it wont release before its finished in the background, so that works as expected.


This is not just for a perticular stick or as stated before; application.


Any ideas?

[Hu]

The file pages were quickly and successfully passed to the kernel to be written to the device. The kernel will write them to the device in the background as time and resources permit. If you need to block until the data is written, call fsync after calling write. This behavior is expected, and occurs on all devices. It is more noticeable on devices that are slow to write.

[ALF__]

[Hu]

I cannot say how the progress bar is judged. If the application is routinely asking the kernel to fsync the data, then after fsync returns, the application can assume that the device has reported that the data was written. Some devices are known to lie about this and merely report it as written when it is transferred to the device's internal cache (as opposed to still being in system RAM). I do not recall how common that behavior is.

[eccerr0r]

Fortunately most devices internal write cache is fairly small, though for like hardware RAID this can be fairly large and who knows when a particular write completes.

I suspect the copy (write) routines can call open() with synchronous writes and thus its status bar should know exactly when a write completes. For the most part writing large files sequentially should be safe, but writing lots of random blocks will limit the disk schedulers what to write first for best performance. Of course this most benefits mechanical disks, but can affect solid state disks as well - the OS can affect fragmentation and metadata writes on SSDs, which can turn into reduced write amplification down the road when it comes time to garbage collect.
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Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?