The idea here is that you can add a flag to a zio, and every vdev that
contributed to the successful completion of that zio will be referenced
on the "trace tree". You can poke around in here from your _done handler
to do any per-vdev followup work.
The actual use case is to track the vdevs that were actually written to,
in order to have a list of vdevs that we should flush. Thats why it
looks like the ZIL vdev flush tracker - the only difference is that it
will also list interior and leaf vdevs, not just toplevel vdevs.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
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