On Sun, Mar 11, 2018 at 11:28:08PM +0700, Andreas Hild wrote:
> Following a physical disk failure of a RAID1 array, I tried to mount
> the remaining volume of a root partition with "-o degraded". For some
> reason it ended up as read-only as described here:
> https://btrfs.wiki.kernel.org/index.php/Gotchas#raid1_volumes_only_mountable_once_RW_if_degraded
> How to precisely do this: dump, recreate and restore its contents?
> Could someone please provided more details how to recover this volume
> safely?

> Linux debian 4.9.0-4-amd64 #1 SMP Debian 4.9.65-3 (2017-12-03) x86_64 
> GNU/Linux

> [ 1313.279140] BTRFS warning (device sdb2): missing devices (1)
> exceeds the limit (0), writeable mount is not allowed

I'd recommend instead going with kernel 4.14 or newer (available in
stretch-backports), which handles this case well without the need to
restore.  If there's no actual data loss (there shouldn't be, it's RAID1
with only a single device missing), you can mount degraded normally, then
balance the data onto the new disk.

Recovery with 4.9 is unpleasant.

⣾⠁⢠⠒⠀⣿⡁ A dumb species has no way to open a tuna can.
⢿⡄⠘⠷⠚⠋⠀ A smart species invents a can opener.
⠈⠳⣄⠀⠀⠀⠀ A master species delegates.
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