iocost.conf — Configuration files for the iocost solution manager
/etc/systemd/iocost.conf
/etc/systemd/iocost.conf.d/*.conf
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.
The default configuration is set during compilation, so configuration is only needed when it is
necessary to deviate from those defaults. The main configuration file is loaded from one of the
listed directories in order of priority, only the first file found is used:
/etc/systemd/,
/run/systemd/,
/usr/local/lib/systemd/ [1],
/usr/lib/systemd/.
The vendor version of the file contains commented out entries showing the defaults as a guide to the
administrator. Local overrides can also be created by creating drop-ins, as described below. The main
configuration file can also be edited for this purpose (or a copy in /etc/ if it's
shipped under /usr/), however 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. This also defines a concept of drop-in priorities to allow OS vendors to ship
drop-ins within a specific range lower than the range used by users. This should lower the risk of
package drop-ins overriding accidentally drop-ins defined by users. It is recommended to use the range
10-40 for drop-ins in /usr/ and the range 60-90 for drop-ins in
/etc/ and /run/, to make sure that local and transient drop-ins
take priority over drop-ins shipped by the OS vendor.
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:
TargetSolution=¶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.
E.g. "TargetSolution=isolated-bandwidth".
udevadm(8), The iocost-benchmarks github project, The resctl-bench documentation details how the values are obtained
[1] 💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If
/usr/local/ is a separate partition, it may not be available during early boot,
and must not be used for configuration.