Re: replacing failed disks in RAID-1 (kernel BUG)?

[Tomasz Chmielewski <mangoo@wpkg.org>]

David Woodhouse wrote:
> On Mon, 2009-07-13 at 12:28 +0200, Tomasz Chmielewski wrote:
>> How do I replace failed disks in RAID-1 mode?
> 
> I don't think you can. In theory you can remove the broken one, and you
> can add a _new_ empty one -- I say 'in theory' because you seem to have
> demonstrated both of those actions failing.
> 
> But I don't believe we have yet implemented anything to let you
> _replace_ a failed disk and recreate its original contents.
> 
> I had that on my TODO list for some time after I get the basic RAID[56]
> operation working.

It would be also interesting to have a tool to monitor the state of the 
RAID (i.e. similar to what /proc/mdstat provides for md).


I also tried to compare what happens when we do writes to md-raid and to 
btrfs-raid (RAID-1 in both cases) and it looks... strange for btrfs. Or 
perhaps this is how RAID-1 works in btrfs?

I used iostat to monitor the writes on both devices.


With md RAID-1, when we do:

# dd if=/dev/zero of=/mnt/md-raid-1/testfile

and

# iostat -dk 1


We can see the write speed on both devices is more or less the same.


With btrfs RAID-1, when we do the same, I can see that writes go to one 
drive, while the second drive receives 0 kb/s writes; then it changes 
(one drive is written to, the second isn't). Only sometimes, writes 
happen concurrently to both drives, like with md RAID-1.

Is it intended?


-- 
Tomasz Chmielewski
http://wpkg.org