NAME

       iocost.conf - Configuration files for the iocost solution manager

SYNOPSIS

       /etc/systemd/iocost.conf /etc/systemd/iocost.conf.d/*.conf

DESCRIPTION

       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[1] 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. 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/ [2], /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.

OPTIONS

       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".

           Added in version 254.

SEE ALSO

       udevadm(8), The iocost-benchmarks github project[1], The resctl-bench documentation details how the
       values are obtained[3]

NOTES

        1. iocost-benchmark
           https://github.com/iocost-benchmark/iocost-benchmarks

        2. 💣💥🧨💥💥💣 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.

        3. The resctl-bench documentation details how the values are obtained
           https://github.com/facebookexperimental/resctl-demo/tree/main/resctl-bench/doc