mdadm: flag device as write-mostly on pre-existing array
 

The title says it all...
I have mdadm managing a RAID1 array between IDE partitions and USB. I want to flag the USB partitions as --write-mostly but don't want to resync my array.. I haven't tried it yet but the man says it works for --build --create and --add... what about a --grow or --re-add? Is it possible to mark the device as write-mostly after the initial creation?

Thanks!
Pat

echo -writemostly >/sys/block/...
 

I had the same problem and I found this solution:
echo -writemostly >/sys/block/md127/md/dev-md2/state
I found it at https://bugs.debian.org/cgi-bin/bugr...cgi?bug=628668 .
It works for me. Context: Ubuntu 16.04, RAID1, one internal HD and other external USB 3.