Re: Volume Managers in Linux

Matthias Urlichs (smurf@noris.de)
4 Nov 1998 16:06:23 +0100


"Theodore Y. Ts'o" <tytso@MIT.EDU> writes:
>
> they can use MD (or LVM) today. But there are advantages to getting the
> filesystem involved; the filesystem can more efficiently handle
> placement issues, and it can more easily handle evacuating a disk used
> in the middle of a logical volume.
>
Evacuating a block in the middle of a file system ought to boil down to
shrinking the file system, copying the middle N chunks to the newly-freed
end chunks, and remapping the thing.

This may be somewhat inefficient, but file system shrinkage should be
fairly easy to do (CERTAINLY easier than allowing arbitrarily-big holes in
the middle).

NB, this assumes that the LVM keeps all chunks of equal size, but anybody
who doesn't do things that was is out of their minds anyway. ;-)

> It's certainly simpler and more intellectually satisfying to say that
> all of these issues should be completely handled in an "lower layer',
> but STREAMS made the same argument about networking, as you may recall.
> Sometimes the most satisfying abstraction boundaries don't result in the
> most efficient and performant implementation.
>
That being said, IMHO we should first do things the layered way and _then_
check if we run into performance problems. As a second approximation, I'd
say that the optimal laying-out of a file system should be mkfs's job, and
doesn't really belong in the actual file system code.

-- 
Matthias Urlichs  |  noris network GmbH   |   smurf@noris.de  |  ICQ: 20193661
The quote was selected randomly. Really.    |      http://www.noris.de/~smurf/
-- 
I have already given two cousins to the war and I stand ready to sacrifice
my wife's brother.
	-- Artemus Ward (1834-1867)

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