A ZFS pool is degraded or gone: is the data lost
A degraded pool does not mean lost data, it means lost redundancy. A pool that seems to have vanished has usually not been deleted either, only detached from the system.
AtlasPVE ·
This entry answers
- zfs pool degraded
- proxmox zfs pool disappeared
- zpool import not working
- accidentally destroyed a zfs pool
- what to do when a zfs disk fails
Two different fears get mixed up: a pool showing as degraded, and a pool not showing at all. They are separate situations, and in both the data is usually still there.
What degraded means
Degraded means the pool has lost one of its members. A pool built as a mirror or with parity keeps running in that state and still serves the data; what was lost is not data but redundancy. The pool is now unprotected against the next failure. So a degraded pool is urgent but not a reason to panic: the urgency is replacing the drive before a second one goes.
Why it degraded
The cause is not always a dead drive. A loose cable, a write cut short during a power loss, or a controller problem produce the same picture. So before swapping a drive, check whether the drive really is the one that left; replacing a healthy drive costs time and money for nothing.
If the pool does not show at all
A pool that is not visible on the system has usually not been deleted. ZFS pools live in a label on the disk itself; when a pool is detached or the configuration is lost, that label stays where it is. In most cases the pool can therefore be brought back. The same holds for a stopped mdadm array and an unmounted Btrfs filesystem.
What not to do
The most dangerous move here is to format the disk or build a new pool on top of it. Only then does the label truly go. The order is this: first check whether the drive is being read at all, then try bringing the pool back, and write nothing to the disk at any step.
Sources
Proxmox's own documentation. In English, and it has the final word on this subject.