Re: RFC - new raid superblock layout for md driver

( sorrt for the delay in replying, I had a week off, and then a week
catching up...)

On Wednesday November 20, sdake@mvista.com wrote:

The only application where having a RAID volume shareable between two 
hosts is useful is for a clustering filesystem (GFS comes to mind). 
 Since RAID is an important need for GFS (if a disk node fails, you 
don't want ot loose the entire filesystem as you would on GFS) this 
possibility may be worth exploring.

Since GFS isn't GPL at this point and OpenGFS needs alot of work, I've 
not spent the time looking at it.

Neil have you thought about sharing an active volume between two hosts 
and what sort of support would be needed in the superblock?

I think that the only way shared access could work is if different
hosts controlled different slices of the device.  The hosts would have
to some-how negotiate and record who was managing which bit.  It is
quite appropriate that this information be stored on the raid array,
and quite possibly in a superblock.  But I think that this is a
sufficiently major departure from how md/raid normally works that I
would want it to go in a secondary superblock.
There is 60K free at the end of each device on an MD array.  Whoever
was implementing this scheme could just have a flag in the main
superblock to say "there is a secondary superblock" and then read the
info about who owns what from somewhere in that extra 60K

So in short, I think the metadata needed for this sort of thing is
sufficiently large and sufficiently unknown that I wouldn't make any
allowance for it in the primary superblock.

Does that sound reasonable?

NeilBrown