[linux-lvm] Simulating LVM Mirror Failure and recovery

[vu pham]

Tomasz Chmielewski wrote:
> vu pham schrieb:
> 
>>> as per the redhat guide this should result in mirror leg failure and 
>>> any subsequent write activity to the mirror should convert the mirror 
>>> into linear volume. but the output of lvs command still shows copy % 
>>> as 100%. also data in filesystem of lv1 is erased and it is converted 
>>> into read only filesystem.
>>>
>>> can anyone please let us know the correct way to simulate lvm mirror 
>>> failure and recovery.
>>>
>>> thanks and regards
>>
>> I think to simulate error failure you have to write to the underlying 
>> devices, not the mirrored device. When you write to the mirrored 
>> device, lvl1 in this case, data is mirrored so there is no failure at 
>> all. So I think you have to dd to either /dev/sda3 or /dev/sdb3.
> 
> How will LVM distinguish between a valid and invalid copy in that case?
> 
> Are you sure that on reads, LVM will return data from the "good" copy, 
> not the one you just overwrote with "dd if=/dev/zero of=/dev/sdb 
> seek=xxx bs=1M count=10"?

Good question. I thought that dd will destroy the lvm information at the 
beginning of the partition which makes the leg broken. I made the 
following test and the result is odd:

- created mirrored logical volumn mrlv1 on /dev/sdc1 and /dev/sdd1, 
having /dev/sdd2 as log device
- mkfs.ext3 /dev/vg1/mrlv1, mount it and copy files onto it.
- dd if=/dev/zero of=/dev/sdd1
- read / write onto the mounted mirror lvm.

The output of lvs  shows:
- lvm information on /dev/sdd2 is destroyed
- the 2nd mirror, which is /dev/sdd2, is *unknown device*
- the mirror is still ok

And no lvm log in /var/log/messages.

[root at xen3 ~]# lvs -a -o +devices
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   Couldn't find device with uuid 'ZUOCeZ-Ssb2-0fP6-n5YY-sz5n-TtXe-b7eL3a'.
   LV               VG   Attr   LSize   Origin Snap%  Move Log
[... deleted unrelated devices ...]

   mrlv1            vg1  mwi-ao 200.00M                    mrlv1_mlog 
100.00         mrlv1_mimage_0(0),mrlv1_mimage_1(0)
   [mrlv1_mimage_0] vg1  iwi-ao 200.00M 
              /dev/sdc1(0)
   [mrlv1_mimage_1] vg1  iwi-ao 200.00M 
              unknown device(0)
   [mrlv1_mlog]     vg1  lwi-ao   4.00M 
              /dev/sdd2(0)



Vu