Dealing with a degraded btrfs array due to disk failure

Forcing a btrfs filesystem to be mounted even though some drives are missing (in a default multi-disk setup, ie. RAID0 for data but RAID1 for metadata):

mount -o degraded,ro /path/to/mount

This assumes that the mounting configuration is defined in your fstab, and will mount it as read-only in a degraded state. You will be able to browse the filesystem, but any file contents may have unexplained gaps and/or be corrupted. Mostly useful to figure out what data used to be on a degraded filesystem.

Never mount a degraded filesystem as read-write unless you have a very specific reason to need it, and you understand the risks. If applications are allowed to write to it, they can very easily make the data corruption worse, and reduce your chances of data recovery to zero!