Re: Question: raid1 behaviour on failure

[Anand Jain <anand.jain@oracle.com>]

On 04/21/2016 01:15 PM, Matthias Bodenbinder wrote:
> Am 20.04.2016 um 15:32 schrieb Anand Jain:
>>> 1. mount the raid1 (2 disc with different size)
>>
>>> 2. unplug the biggest drive (hotplug)
>>
>>    Btrfs won't know that you have plugged-out a disk.
>>    Though it experiences IO failures, it won't close the bdev.
>
> Well, as far as I can tell mdadm can handle this use case. I tested that. I have an mdadm raid5 running. I accidentially unplugged a sata cable from one of the devices and the raid still worked. I did not even notice that the cable was unplugged until a few hours later. Then I plugged in the cable agaib and that was it. mdadm recovered the raid5 without any problem. -> This is redunancy!

  Yep. I meant to say its a bug in btrfs that it won't know
  about the missing device (after mount). Pls do test the hot
  spare patch set it has few first steps (yep not a complete)
  to handle the failed device while FS is mounted.


>>> 3. try to copy something to the degraded raid1
>>
>>    This will work as long as you do _not_ run unmount/mount.
>
> I did not umount the raid1 when I tried to copy something. As you can see from the sequence of events: I removed the drive and immdiately afterwards tried to copy something to the degraded array. This copy failed with a crash of the btrfs module. -> This is NOT redundancy.
>
> The ummount and mount operations are coming afterwards.
>
> In a nutshell I have to say that the btrfs behaviour is by no means compliant with my understanding of redundancy.

  A known issue.
  Your testing / validating of hot spare patch set will help.

Thanks, Anand