Is it possible to grow a RAID 10 array? Can I take it offline and grow it? Or do I have to make an entirely new array, and copy all the data onto it?
As far as I know, it’s possible, but you have to mess with the superblock. I have seen solutions that simulate RAID10 by creating multiple RAID1 arrays and use LVM to effectively RAID0 the RAID1 arrays, thus creating a hybrid RAID10. I have not tried it myself, but it seems like a good solution. It also allows you to know exactly which disks can be safely failed out, something that is difficult to discern with md.