feature request: btrfs redundancy report

[CHerSun]

I'd really appreciate some way to cleanly get blocks redundancy state.

Example:

• 4 drives in d=RAID1, m=RAID1c3
• 1 drive went missing

lsblk - reports 3 drives now.
btrfs device scan - reports 3 devices with no other notices
btrfs device usage ... - reports all 4 drives with no marks really, only device size = 0
btrfs filesystem show ... - shows all 4 drives, but marks drive as missing.
btrfs filesystem usage -T ... - shows all devices without marking them in any way (with missing device value > 0).
btrfs filesystem df ... - may report or may not report *some devices are missing. Looks like that depends on which drive went missing, not sure.

Judging by that - btrfs knows it does miss a device, but it doesn't do the next step - to notify about redundancy problem in a clear way, so that end user could take a proper action, if needed (in my case btrfs device delete <devid> until I can replace). What I mean is that I'd really like to see something like btrfs filesystem status ... with statements like:

• Online/Degraded
• Physical devices: 3 out of 4
• Data state: ### blocks in RAID1 have NO redundancy
• Metadata state: redundancy is reduced

Or:

• Online/healthy
• Physical devices: 4 out of 4
• Data state: healthy
• Metadata state: healthy

Or (hypothetically):

• Offline/Failed
• Physical devices: 3 out of 4
• Data state: ### blocks in RAID0 were lost
• Metadata state: redundancy is reduced

Is that possible?