systemd-udev-settle.service - Wait for all pending udev
          events to be handled


          This service calls udevadm settle to wait until all events
          that have been queued by udev(7) have been processed. It is
          a crude way to wait until "all" hardware has been
          discovered. Services may pull in this service and order
          themselves after it to wait for the udev queue to be empty.

          Using this service is not recommended.  There can be no
          guarantee that hardware is fully discovered at any specific
          time, because the kernel does hardware detection
          asynchronously, and certain buses and devices take a very
          long time to become ready, and also additional hardware may
          be plugged in at any time. Instead, services should
          subscribe to udev events and react to any new hardware as it
          is discovered. Services that, based on configuration, expect
          certain devices to appear, may warn or report failure after
          a timeout. This timeout should be tailored to the hardware
          type. Waiting for systemd-udev-settle.service usually slows
          boot significantly, because it means waiting for all
          unrelated events too.

          udev(7), udevadm(8)

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