On Thursday, December 26, 2019 2:54:25 PM MST Chris Murphy wrote:
On Tue, Dec 24, 2019 at 2:56 PM Cameron Simpson <cs(a)cskk.id.au>
wrote:
>
>
> Oh yes, one more thing. If you do the RAID1 thing: either make a shiny
> new RAID1 and copy to it, or practice the transition with test drives.
> Do not risk your high value data by trying to "in place RAID1" it.
I'm not sure if this is even possible with mdadm or lvm. For sure they
have no way of knowing which mirror is correct. But even if it's
possible, it's a bit complicated because it implies repartitioning in
order to make room for the necessary metadata area.
Whereas converting a Btrfs volume from single to raid1 uses balance
code, so it isn't any riskier than a full balance.
It is possible with mdadm, and is even easier with lvm as you're just
converting to a mirror. With mdadm, you'd create the RAID1 with 'missing' for
the first disk, and the disk itself for the 2nd. It's not risky if you do it
this way.