Re: Attempt to change raid1 to raid0 results in division error in kernel

[Robert Goliasz <r.goliasz@digital-science.com>]

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Hi Stan,

Thanks for your reply.

The point of my thread was to point out that a userland tool shouldn't
be able to cause a kernel to crash. It also shouldn't segfault when given
incorrect arguments.

If conversion from raid1 to raid0 is supported but I didn't specify
the right arguments, I should get an appropriate error message. If it's
not supported, then I should also get an appropriate error message.
My machine shouldn't crash because of me giving wrong arguments to mdadm.

Another, smaller, issue is that if conversion from raid 1 to raid 0 is
not supported then perhaps the mdadm manpage should be clearer about it,
rather saying "Currently supported growth options [...] [are] changing the
RAID level between 0, 1, 5, and 6"

>>   gbl-macbook# mdadm --grow /dev/md0 -l 0
>If it is possible, this command line won't work because you didn't
>specify a chunk size for the RAID0 stripe.  Every striped array type
>requires a chunk size.

This doesn't seem to be correct, I'm able to at least create a RAID0 stripe
without specifying the chunk size explicitly:

  root@gbl-macbook:~# mdadm --create /dev/md1 -l 0 -n 2 /dev/vg0/v1 /dev/vg0/v2
  mdadm: /dev/vg0/v1 appears to be part of a raid array:
      level=raid1 devices=3 ctime=Thu May 23 11:13:33 2013
  Continue creating array? y
  mdadm: Defaulting to version 1.2 metadata
  mdadm: array /dev/md1 started.
  root@gbl-macbook:~# cat /proc/mdstat 
  Personalities : [raid0] 
  md1 : active raid0 dm-4[1] dm-3[0]
        203776 blocks super 1.2 512k chunks
        
  unused devices: <none>
  root@gbl-macbook:~# 


>> Afterwards, my mounted filesystem (/dev/md0) disappeared (it's no longer
>> mounted), and all operations related to software raid seem to fail:
>Well of course.  Experimenting with mdadm without knowing what you're
>doing will often result in lost/corrupted arrays and other forms of damage.

I knew exactly what I was doing. I was testing whether conversion from raid1
to raid0 was supported. I got an unexpected result in the form of a kernel
crash. After discussing on IRC I was asked to report this here.

I've concluded that it's not supported or broken and will not attempt to run
this on a production machine.

-- 
Robert Goliasz
Infrastructure Engineer

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