Re: [Evms-devel] Possible to create a degraded RAID5 array with EVMS?

[Mike T. <mh...@us...>]

James Lee wrote:

>> From: Mike Tran <mh...@us...>
>> To: evm...@li...
>> Subject: Re: [Evms-devel] Possible to create a degraded RAID5 array 
>> with    EVMS?
>> Date: Tue, 29 Nov 2005 18:50:34 -0600
>>
>> Mike Tran wrote:
>>
>>> On Sat, 2005-11-26 at 14:42, James Lee wrote:
>>>
>>>
>>>> Hi there,
>>>>
>>>> I'm having some more trouble with getting EVMS and mdadm to play 
>>>> nicely together (even after upgrading EVMS)...
>>>>
>>>> The steps I'm taking are:
>>>>
>>>> 1. Starting with an empty drive (wiped it by zero-filling the start 
>>>> and end of drive to make sure there's no residual partition table 
>>>> information).  Create two logical partitions (/dev/sdb5 and 
>>>> /dev/sdb6).
>>>>
>>>> 2. Use mdadm to create a degraded "3-drive" RAID5 array called 
>>>> /dev/md0: "mdadm --create /dev/md0 --level=5 --raid-devices=3 
>>>> /dev/sdb5 /dev/sdb6 missing".
>>>>
>>>> 3. Start EVMS.  It correctly detects the degraded RAID5 array on 
>>>> /dev/md0.
>>>>
>>>> 4. In EVMS, create 4 partitions on /dev/sdb, each one just over 
>>>> half the size of the partitions in the RAID5 array.  Create two 
>>>> 2-drive RAID0 stripes, from these partitions.
>>>>
>>>> 5. Add one of these RAID0 arrays to the RAID5 array.  Wait for it 
>>>> to resync.  RAID5 array is now active and non-degraded.
>>>>
>>>> 6. Create an EXT3 filesystem and bung some files on it.
>>>>
>>>> 7. Everything working fine so far.  Now expand the RAID5 array with 
>>>> the other RAID0 array.  This seems to work fine.  No data lost on 
>>>> the partition and no errors.
>>>>
>>>> 8. Reboot.  When next starting EVMS, I get the following errors:
>>>>
>>>>
>>>>
>>>
>>> In theory, this should work!!! I will try your scenario to find out 
>>> what
>>> went wrong.
>>>
>>>
>>>
>> I could not reproduce this problem using evms 2.5.3.  Did you wait 
>> for the raid5 expand to complete before rebooting the machine?
>>
>> -- 
>> Mike T.
>>
>
> Thanks for looking into this Mike.  Yes, the RAID5 expand had 
> completed successfully (and the machine was idle for a few hours 
> before rebooting, with the array working fine).
>
> Is it possible that these problems are caused by having some residual 
> superblock left over from a previous array?  To save time, I wiped the 
> drive by doing a wipe of the first and last million sectors of the 
> drive (rather than zeroing the entire 320GB, which takes several 
> hours).  Maybe I should try with a completely clean drive.

I don't think zeroing the entire disk will make any difference.  What 
you did was 100% valid scenario.  You've seen the problem and I want to 
fix it.  If you can reproduce the problem, please let me know.

>
> The version of mdadm I'm using doesn't support version 1 superblocks 
> AFAIK, which is why I've had to use the older version 0.9 
> superblocks.  I can't see myself having more than 27 devices in this 
> array, or moving it over to a byte-swapped (i.e. Sparc?) machine, so I 
> should be OK.  Presumably support for the older superblock will 
> continue into the future? 

Not many people use 1.0 superblock format. The kernel md driver code to 
support the old superblock is relatively small.  Moreover, data is 
important.  Therefore, I believe that the old superblock will be 
supported for a very long time :)

--
Thanks,
Mike T.