usb-storage-optimized-async

Udev rule and systemd service to solve the problem of USB storage devices incomplete data sync during ejecting on Linux.

Problem

0. You're on a Linux PC
1. You write some data to the USB storage device,
   for example, you write Windows ISO on it, so you can install it on another PC later
2. You eject the USB storage device once the ISO writing apparently gets finished
3. You plug the USB storage device on another PC to install Windows on it
4. You notice that Windows doesn't want to load and that data on USB storage device is corrupted

This happens often on Linux and is one of the most frustrating problems that users can face.
This is something that Windows doesn't have an issue with.

Cause

On Linux, USB storage devices use async mount option by default, which enables USB writing cache on RAM.
This speeds up the operation of writing the data to the USB storage device considerably compared to sync.
It's also more efficient in writing the data, writing less data on the storage compared to sync.
However, this is not tuned properly by default, which makes the system incorrectly report that data is fully written too early,
while some of the data is still in the USB writing cache.

As a comparison, Windows enables behavior similar to sync by default for USB storage devices.

Solution

Non-ideal solutions

1. Always manually clicking "Unmount" on USB storage device, waiting for it to finish and then ejecting it.
   This will always guarantee that all the data will be flushed to the USB storage device,
   but this is not as user-friendly as simply ejecting the USB drive.
2. Using sync as a mount option for USB storage device.
   This disables the USB writing cache and solves this problem.
   However, data writing will be much slower and it will increase the tear on USB storage device.

More ideal solution

1. Using this udev rule and systemd service to optimize the async mount option, without sacrificing USB storage speed and its write cycles.

How it works

Udev rule

1. Launch the usb-storage-optimized-async-udev script when USB storage device is detected and supply it with USB device block, USB vendor id and USB model id values
2. It determines the USB port speed of the USB storage device that is plugged into it using lsusb -t -v output
   (systemd's udev has an option to supply ATTRS{speed} for this, but I found it to be inaccurate in my testing)
3. Do the calculation of the ideal BDI max_bytes value for the USB writing cache on the USB storage device based on:

• USB port speed (in MB)
• buffer time (0.05)
• safety factor (1.3)

4. Calculation looks like this:
   ( USB port speed / 8 ) * buffer time * safety factor * 1024 * 1024
5. Enable the data write RAM cache limit (BDI strict_limit) and apply the calculated value to BDI max_bytes file

By lowering and limiting the max_bytes BDI value, we assure that USB storage device won't overuse the USB writing cache.

Systemd service

It does the same as the udev rule, with the difference that it only applies once on boot,
as udev rule can trigger too early during the system boot when USB storage device is already plugged in for it to work.

To find the USB storage block device, as the service script doesn't have that data available from udev as udev script does,
it uses the POSIX function get_usb_storage_block, which parses all USB bus symlinked directories in /sys/bus/usb/devices/ and
checks which of those symlinked directories have matched USB vendor and product ID in idVendor and idProduct file;
when those symlinked directories are found, they are compared to physical /sys/block/$block_device/ directories;
when the physical /sys/block/$block_device/ directory matches to the symlinked directory, that's how the USB storage block device is found.

Logging

Logs for the udev rule and Systemd service for the current boot are stored in:

• /tmp/usb-storage-optimized-async-udev.log
• /tmp/usb-storage-optimized-async-service.log

Requirements

• Linux 6.1+ kernel
  • max_bytes BDI value is only available since then.
  • https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-bdi
• /bin/sh POSIX shell
• udev
  • only tested on SystemD's udev
• lsusb
• gawk
• bc
• Script autostarter on boot (init system like SystemD or something else)
  • for applying the service script fix during later stage of boot, when USB storage device is already plugged in before and during the boot, where udev triggers too early to work

Optional dependency:

• date
  • for adding the time and date info to the log

Known quirk

• If 2 or more USB storage devices have the same USB vendor ID and USB model ID, then this fix won't apply for those USB storage devices.
  This doesn't happen often, but it's worth to note.
  lsusb output doesn't show the information about the USB storage block device, hence why we can't use anything else as an identifier.

Credits

To clarify, I am not the one who discovered this, I'm the one who just refined it to work reliably.

Linux Manjaro uses the different and simpler script compared to mine, which I tested and found it to not work reliably,
however, the maintainer couldn't reproduce the issue.
https://gitlab.manjaro.org/applications/udev-usb-sync

• Linux Manjaro user Megavolt who discovered this method and explained it well with benchmarks.
  • https://forum.manjaro.org/t/strict-limit-of-write-cache-0s-sync-time-policy-for-usb-devices-by-default/166934
• Linux Manjaro and udev-usb-sync script maintainer Frede H who patiently reviewed and answered my emails regarding the script.