Linux filesystem cache
Linux agressively caches files in order to improve overall performance.
When copying large amount of files to an USB storage this often results in some wait time until the device can be safely removed.
How long you have to wait depends on your system and the quality of the USB storage device.
Numerous issues regarding this has resulted in various suggestions involving sysctl tweaks and trim.
Examples:
- Decrease dirty bytes for more reliable USB transfer
- Slow USB transfers
- USB Transfer Speed 5MB/sec & dropping
- Extremely slow file transfer speeds from usb 3.0 to external hdd
- Transfer speed slows down to a crawl after ~ 10GB transfered
- USB transfer data drops drastically and maximum number of clients are reached
- File copy on a usb 2 slows down to a halt
- Dolphin large file transfer to usb stick indicates completed after caching is done, transfer still ongoing
udev rule to disable write-cache
In another place one user threw in an udev rule which would disable write-cache for devices when they were added and while it was just an idea - it triggered my curiosity.
I dug into the intricacies of udev and found a method to only target USB storage devices.
- Documentation for udev - ArchWiki
The rule has gotten a major overhaul and now consist of the rule, a config file and a script
The rule /etc/udev/rules.d/99-usb-sync.rules
# rule to disable write cache for usb storage
# requires hdparm to be installed
ACTION=="add|change", KERNEL=="sd[a-z]", ENV{ID_USB_TYPE}=="disk", RUN+="/usr/bin/hdparm -W 0 /dev/%K"
#
# the following rules is introduced with kernel 6.2
# https://docs.kernel.org/admin-guide/abi-testing.html#abi-sys-class-bdi-bdi-strict-limit
# https://docs.kernel.org/admin-guide/abi-testing.html#abi-sys-class-bdi-bdi-max-ratio
# https://docs.kernel.org/admin-guide/abi-testing.html#abi-sys-class-bdi-bdi-max-bytes
ACTION=="add|change", KERNEL=="sd[a-z]", ENV{ID_USB_TYPE}=="disk", RUN+="/usr/bin/udev-usb-sync %k"
The config /etc/usb-dev-sync/udev-usb-sync.conf
# default values
#use_tweaks=1
#max_bytes=16777216
#max_ratio=50
#strict_limit=1
The script /usr/bin/udev-usb-sync
#!/usr/bin/bash
#
# script to tweak USB storage device filesystem sync
#
# sources /etc/usb-dev-sync/usb-dev-sync.conf
#
use_tweaks=1
max_bytes=16777216
max_ratio=50
strict_limit=1
# read user config
source /etc/udev-usb-sync/udev-usb-sync.conf
if [[ "$use_tweaks" = 0 ]]; then
exit 0
fi
if [[ -z "$1" ]]; then
exit 1
fi
echo "$max_bytes" > "/sys/block/$1/bdi/max_bytes"
echo "$max_ratio" > "/sys/block/$1/bdi/max_ratio"
echo "$strict_limit" > "/sys/block/$1/bdi/strict_limit"
The rule activates when udev detects
- add or change
- kernel event for disk devices sd[a-z]
- only if the device environment ID_USB_TYPE==âdiskâ
- run
- hdparm -W 0 /dev/%k (disable write cache if supported)
- udev-usb-sync %k
- applies defaults
- read config and apply user values
- if use_tweaks=0 the script exits
- if use_tweaks=1 the applies the values (default or config)
Create a file in /etc/udev/rules.d/99-usb-sync.rules and paste the rule into it.
Create a file in /etc/udev-usb-sync/udev-usb-sync.conf and paste the default values.
Create a file in /usr/bin/udev-usb-sync and paste the script content.
Install hdparm package.
sudo pacman -Syu hdparm
Reload udev
sudo udevadm control --reload
Then plug an usb device - open in your file manager - copy a huge amout of files to the device - when the copy is done - click eject in the file manager - note how quick the device is ejected.
For those preferring the package manager, I have created a PKGBUILD which will pull the hdparm dependency upon installation.
pamac build udev-usb-sync
Another fine utility script provided by @cscs fine-tunes a number of system parameters with the option to input your own values when the script is run
- Cross posted - Disable write-cache for USB devices | root.nix.dk
- Related topic with tests - Slow USB transfers - #12 by linux-aarhus
