This file configures the behavior of "iocost", a tool mostly used by systemd-udevd(8) rules to automatically apply I/O cost solutions to /sys/fs/cgroup/io.cost.*.
The qos and model values are calculated based on benchmarks collected on the iocost-benchmark project and turned into a set of solutions that go from most to least isolated. Isolation allows the system to remain responsive in face of high I/O load. Which solutions are available for a device can be queried from the udev metadata attached to it. By default the naive solution is used, which provides the most bandwidth.
Configuration Directories and Precedence
The default configuration is set during compilation, so configuration is only needed when it is necessary to deviate from those defaults. Initially, the main configuration file in /etc/systemd/ contains commented out entries showing the defaults as a guide to the administrator. Local overrides can be created by editing this file or by creating drop-ins, as described below. Using drop-ins for local configuration is recommended over modifications to the main configuration file.
In addition to the "main" configuration file, drop-in configuration snippets are read from /usr/lib/systemd/*.conf.d/, /usr/local/lib/systemd/*.conf.d/, and /etc/systemd/*.conf.d/. Those drop-ins have higher precedence and override the main configuration file. Files in the *.conf.d/ configuration subdirectories are sorted by their filename in lexicographic order, regardless of in which of the subdirectories they reside. When multiple files specify the same option, for options which accept just a single value, the entry in the file sorted last takes precedence, and for options which accept a list of values, entries are collected as they occur in the sorted files.
When packages need to customize the configuration, they can install drop-ins under /usr/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. Drop-ins have to be used to override package drop-ins, since the main configuration file has lower precedence. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering of the files.
To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to /dev/null in the configuration directory in /etc/, with the same filename as the vendor configuration file.
All options are configured in the [IOCost] section:
Chooses which I/O cost solution (identified by named string) should be used for the devices in this system. The known solutions can be queried from the udev metadata attached to the devices. If a device does not have the specified solution, the first one listed in IOCOST_SOLUTIONS is used instead.
udevadm(8), The iocost-benchmarks github project, The resctl-bench documentation details how the values are obtained
- The resctl-bench documentation details how the values are obtained