Re: btrfs as / filesystem in RAID1

[Martin Steigerwald <martin@lichtvoll.de>]

Chris Murphy - 07.02.19, 18:15:
> > So please change the normal behavior
> 
> In the case of no device loss, but device delay, with 'degraded' set
> in fstab you risk a non-deterministic degraded mount. And there is no
> automatic balance (sync) after recovering from a degraded mount. And
> as far as I know there's no automatic transition from degraded to
> normal operation upon later discovery of a previously missing device.
> It's just begging for data loss. That's why it's not the default.
> That's why it's not recommended.

Still the current behavior is not really user-friendly. And does not 
meet expectations that users usually have about how RAID 1 works. I know 
BTRFS RAID 1 is no RAID 1, although it is called like this.

I also somewhat get that with the current state of BTRFS the current 
behavior of not allowing a degraded mount may be better… however… I see 
clearly room for improvement here. And there very likely will be 
discussions like this on this list… until BTRFS acts in a more user 
friendly way here.

I faced this myself during recovery from a failure of one SSD of a dual 
SSD BTRFS RAID 1 and it caused me having to spend *hours* instead of 
what in my eyes could be minutes to recover the machine to a working 
state again. Luckily the SSDs I use do not tend to fail all that often. 
And the Intel SSD 320 that has this "Look, I am 8 MiB big and all your 
data is gone" firmware bug – even with the firmware version that was 
supposed to fix this issue – is out of service now. Although I was able 
to bring it back to a working (but blank) state with a secure erase, I 
am just not going to use such a SSD for anything serious.

Thanks,
-- 
Martin