On Tue, Sep 08, 2015 at 09:18:05PM +0200, Ian Kumlien wrote: > Hi, > > Currently i have a raid1 configuration on two disks where one of them > is failing. > > But since: > btrfs fi df /mnt/disk/ > Data, RAID1: total=858.00GiB, used=638.16GiB > Data, single: total=1.00GiB, used=256.00KiB > System, RAID1: total=32.00MiB, used=132.00KiB > Metadata, RAID1: total=4.00GiB, used=1.21GiB > GlobalReserve, single: total=412.00MiB, used=0.00B > > There should be no problem in failing one disk... Or so i thought! > > btrfs dev delete /dev/sdb2 /mnt/disk/ > ERROR: error removing the device '/dev/sdb2' - unable to go below two > devices on raid1
dev delete is more like a reshaping operation in mdadm: it tries to
remove a device safely whilst retaining all of the redundancy
guarantees. You can't go down to one device with RAID-1 and still keep
the redundancy.
dev delete is really for managed device removal under non-failure
conditions, not for error recovery.
> And i can't issue rebalance either since it will tell me about errors
> until the failing disk dies.
>
> Whats even more interesting is that i can't mount just the working
> disk - ie if the other disk
> *has* failed and is inaccessible... though, i haven't tried physically
> removing it...
Physically removing it is the way to go (or disabling it using echo
offline >/sys/block/sda/device/state). Once you've done that, you can
mount the degraded FS with -odegraded, then either add a new device
and balance to restore the RAID-1, or balance with
-{d,m}convert=single to drop the redundancy to single.
> mdam has fail and remove, I assume for this reason - perhaps it's
> something that should be added?
I think there should be a btrfs dev drop, which is the fail-like
operation: tell the FS that a device is useless, and should be dropped
from the array, so the FS doesn't keep trying to write to it. That's
not implemented yet, though.
Hugo.
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