Re: RFC - new raid superblock layout for md driver

From: Neil Brown (
Date: Sun Dec 08 2002 - 17:35:06 EST

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

On Wednesday November 20, 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?

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This archive was generated by hypermail 2b29 : Sun Dec 15 2002 - 22:00:13 EST