[dm-devel] [PATCH v3 0/8] dm-raid (raid456) target

[Jonathan Brassow]

On Jan 9, 2011, at 6:37 PM, NeilBrown wrote:

>>> Also:
>>>
>>> 3:	<#raid_devs> <meta_dev1> <dev1> .. <meta_devN> <devN>
>>>
>>> This doesn't allow offsets into the various devices like dm-stripe,
>>> dm-linear, dm-raid1 all do.
>>
>> This is a conscious choice.  The offset was discarded because it is
>> unneeded.
>
> Famous last words!!!

:)

>
> When reshaping a RAID5/6 to change the chunk size or layout, or when
> converting an N drive RAID5 to N+1 drive RAID6, it makes it a lot  
> easier if
> you can can change the data offset at the same time.  Then you don't  
> have to
> keep backing up data.  md cannot do this yet but I hope to add the  
> code soon
> and because 'data_offset' is a distinct value it is conceptually  
> easy to do.
> If 'data_offset' is assumed to be zero, then it isn't.

I guess I don't understand this part.  Why would you need to change  
the data offset?  Especially the offset as it relates to the  
individual drives?

> But I don't know to what extent you are planning on supporting  
> reshaping of
> arrays....

I don't see a reason why this can't be completely supported...

  brassow