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