It shoulds to me like you need to discover write-intent bitmaps.
Such a bitmap is effectively a set of 'dirty' bits, one for each chunk of the array (and you can choose the chunk size).
So if you set the chunk size to 50GB (I would probably set it a bit smaller) you get the same functionality as you describe, only with much less hassle.
So just create a raid1 or - if you have more than 2 drives - raid10, and
mdadm --grow /dev/md0 --bitmap=internal --bitmap-chunk=1000000and you will be much happier.
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