That's a great report, thank you!  There is a tunable called 'md_sync_thresh' which you can set with a command such as:   mdcmd set md_sync_thresh <N>   The value <N> is a count of 'stripes' from 0..md_sync_window.  The way it works is: when a parity sync/check starts, it queues up 'md_sync_window' number of 'stripes'.  Thus the maximum number of in-progress 'stripes' is md_sync_window.  As these 'stripes' complete the number in-progress obviously starts decreasing.  When it re