Re: Roadmap for md/raid ???

[Neil Brown <neilb@suse.de>]

On Monday January 19, davidsen@tmr.com wrote:
> Neil Brown wrote:
> >
> > The difficulty is "How do you handle a crash?"
> > If you crash and restart in the middle of a reshape, you need to know
> > where all the data is.
> > To follow your scheme, you would need to update the metadata for
> > every block that is moved.
> >
> >   
> That would seem to be an issue doing it in a large task as well, if I 
> understand what you want to do. After the data have been written to the 
> spare space I believe you are going to write it back. If you update the 
> metadata before the write and something happens you have old metadata 
> but data in another format. Conversely, if you update the metadata 
> first, and something happens, you have the metadata describing a change 
> which didn't happen. So You would have to have a metadata reflecting the 
> data not being in the array, but actually somewhere else, or you would 
> have to keep the spare space inside the array, where room might not be 
> available.

I "freeze" a section of the array so that any writes block,
I copy that data to somewhere safe, and mark that copy as being a
valid copy of the moved data.
Then I copy it back in the new layout and update the array metadata to
show that the change-over point between old-layout and new-layout has
changed.  Then I invalidate the copy.
Then repeat.

When restarting an array, mdadm checks the copy (which could be in a
file on a different device) and if it is valid, it is copied on to
the array and the metadata is updated.

NeilBrown