On 2019-04-28 16:14, Andrei Borzenkov wrote:
28.04.2019 22:35, Hendrik Friedel пишет:
Hello,
I intend to move to BTRFS and of course I have some data already.
I currently have several single 4TB drives and I would like to move the
Data onto new drives (2*8TB). I need no raid, as I prefer a backup.
Nevertheless, having raid nice for availability. So why not in the end.
I currently use ~6TB, so it may work, but I would be able to remove the
redundancy later.
So, if I understand correctly, today I want
-m raid1 -d raid1
whereas later, I want
-m raid1 -d single
What is very important to me is, that with one failing drive, I have no
risk of losing the whole filesystem, but only losing the affected drive.
Is that possible with both of these variants?
With "single" data profile you won't lose filesystem, but you will
irretrievably lose any data on the missing drive. Also "single" profile
does not support auto-healing (repairing of bad copy from good copy). If
this is acceptable to you, then yes, both variants will do what you want.
Actually, it's a bit worse than this potentially. You may lose
individual files if you lose one disk with the proposed setup, but you
may also lose _parts_ of individual files, especially if you have lots
of large (>1-5GB in size) files. And on top of this, finding what data
went missing will essentially require trying to read every byte of every
file in the volume.
Is it possible to move between the two (doing a balance, of course?
Yes as long as you have sufficient free space for target profile.
Any other thoughts/recommendations?
As of today there is no provision for automatic mounting of incomplete
multi-device btrfs in degraded mode. Actually, with systemd it is flat
impossible to mount incomplete btrfs because standard framework only
proceeds to mount it after all devices have been seen. As long as you do
not use systemd in initramfs you may be able to boot by passing suitable
root mount flags on kernel command line.
