Re: Migration to BTRFS

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

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.
>