Synology SHR Recovery: LVM Under Btrfs
On Synology, SHR combines md RAID devices into an LVM volume group, then Btrfs sits on top. Three layers mean three possible failure points.
Layer order matters
md RAID assembles members into /dev/mdX; LVM maps those into a volume group and logical volume; Btrfs consumes the logical volume. A 'volume crashed' message can come from any of the three.
Recovery approach
Image every member read-only, reassemble md RAID virtually with the recorded parameters, then activate LVM metadata, then mount Btrfs (or parse it if it will not mount). Never run 'repair' on the live volume.
FAQ
Can I read a single drive from SHR?
No — data and metadata are distributed across members.
Does SHR-2 survive two failures?
It is designed to, assuming both members can be imaged.