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

quoted

quoted

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