Re: Linear device of two arrays

[NeilBrown <neilb@suse.com>]

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On Fri, Jul 14 2017, Veljko wrote:

> On 07/14/2017 03:57 AM, NeilBrown wrote:
>>
>> Presumably you also have an md3 raid10 which you want to attach to the
>> end of md2?
>
> Yes, I just created it. It's resyncing.
>
>
>> md2 is 5761631232 sectors.
>>   2880815616 kilobytes
>>   2813296.5 (binary)megabytes.
>>
>> When you include that into a "linear" you will lose a few K from the
>> end.
>> It might be sensible to cause the "linear" to use whole stripes from
>> the raid10, where a stripe is 1M (2 512K chunks).
>> If you did that, you would lose a little over 1M.
>> So backup the last 3.5 M of the raid10.  This is much more than you need.
>>
>> ie.
>>
>>   dd if=/dev/md2 of=SOMEWHERE/SAFE bs=1M skip=2813293
>
> I'm little confused. What I'm backuping is last 3.5M of used space, 
> right? How is that only ~2.7T? df shows 4.8T of used space.

You were right to check.

md2 is 5761631232 kilobytes, not sectors.
so 5626593 (binary) megabytes (exactly)

So command should be

>>   dd if=/dev/md2 of=SOMEWHERE/SAFE bs=1M skip=5626590

and expect it to create a 3M file.

Use this 'skip' number of the 'seek' number later.

NeilBrown


>
> Rest of the instructions are clear and I'll try it as soon as md3 is 
> synced.
>
> Thanks Neil!
>
>> (dd treats 'M' as 1024*1024, MB is 1000*1000)
>>
>> If the file this creates is not 3.5M, then something went wrong.  Stop
>> here.
>>
>> Just to be safe you might want to backup the first few megabytes.  You
>> won't need this unless something goes wrong
>>   dd if=/dev/md2 of=SOMEWHERE/ELSE bs=1M count=10
>>
>> Now create the linear from /dev/md2 and /dev/md3(?).  Be sure to use
>> "-e 1.0 --data-offset=0".  This creates /dev/md4
>>
>> Now restore the first backup
>>
>>  dd if=SOMEWHERE/SAFE of=/dev/md4 bs=1M seek=2813293
>>
>> Be sure to use the same bs= and seek= as you did the first time.
>> Be sure it is copying from the back and to the new linear raid.
>>
>> You should now be done. Check your xfs filesystem, and maybe even mount
>> it and use it.
>>
>> NeilBrown
>>
>>>
>>> # mdadm --detail /dev/md2
>>> /dev/md2:
>>>          Version : 1.2
>>>    Creation Time : Fri Sep 14 12:40:13 2012
>>>       Raid Level : raid10
>>>       Array Size : 5761631232 (5494.72 GiB 5899.91 GB)
>>>    Used Dev Size : 2880815616 (2747.36 GiB 2949.96 GB)
>>>     Raid Devices : 4
>>>    Total Devices : 4
>>>      Persistence : Superblock is persistent
>>>
>>>      Update Time : Mon Jul 10 12:32:51 2017
>>>            State : clean
>>>   Active Devices : 4
>>> Working Devices : 4
>>>   Failed Devices : 0
>>>    Spare Devices : 0
>>>
>>>           Layout : near=2
>>>       Chunk Size : 512K
>>>
>
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