Re: Specifying a data offset?

[Phil Turmel <philip@turmel.org>]

On 05/08/2013 04:17 PM, Tregaron Bayly wrote:
> We have lots of mirrors all over our network and over time this means
> arrays created by different versions of mdadm.  In turn this means that
> we have all sorts of data offsets, including a 2.3.5 mdadm version that
> offset the data 262,144 sectors.  I can't fathom why I need my data to
> start 128M from the beginning of the disk, but whatever.
> 
> Inevitably an admin decided that doing a --create over the top of an
> existing array was a good and necessary thing and when finished we
> couldn't find the filesystem.  Bringing it back online was simply a case
> of finding the correct offset, locating an mdadm version that would
> specify it properly and doing another create.  Through the whole
> exercise I found myself wondering why there isn't a command line
> argument to specify the data offset if something other than the default
> is desired.  I think that implementing the argument and reading the
> value in init_super wouldn't be terribly difficult so I must be missing
> an obvious reason.  Can someone explain to me why this feature isn't
> desirable?

This desirable and has been implemented.  Check out the development
version of mdadm and look at the "--data-offset" option.

It does not exist in a released version yet, but it has been cooking
since October.

Phil