# iocost.conf

## 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
           is 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.
    


## 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
    


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