hybrid SDD/HDD raid 1

 Viz https://www.tansi.org/hybrid/

The key is the option "write-mostly" passed to hdd.

 

mdadm --metadata=0.90 --create -n 2 -l 1 /dev/md1 /dev/ssd -W /dev/hdd

For an existing array, add a new ssd (after ssd failure)

mdadm --manage /dev/md0 --add /dev/sda1

--write-mostly is a per-member setting stored in the md metadata for that member device (and also maintained in-kernel while the device is in the array). It should survive on the hdd.

quote: You can enable "write-mostly" for a RAID component in the following way:

echo writemostly >  /sys/block/md0/md/dev-hdd/state

and disable it this way:

echo -writemostly >  /sys/block/md0/md/dev-hdd/state

(nicmene, po pridani noveho ssd disku se ve vypisu vlastnosti /dev/hdd write mostly ukazovalo. Teprve az po echo writemostly > ...)