Specifying a data offset?

Specifying a data offset?

[Tregaron Bayly]

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?

Thanks,

Tregaron

Re: Specifying a data offset?

[Phil Turmel]

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

Re: Specifying a data offset?

[Tregaron Bayly]

On Wed, 2013-05-08 at 16:44 -0400, Phil Turmel wrote:

> 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.

Ah, very nice.  Thanks!