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    SubjectRe: [Evms-devel] Re: Linux v2.5.42
    On Sun, Oct 13, 2002 at 11:16:24PM +0800, Michael Clark wrote:
    > On 10/13/02 21:49, Christoph Hellwig wrote:
    > > On Sun, Oct 13, 2002 at 08:41:20PM +0800, Michael Clark wrote:
    > >
    > >>Exactly. I think Christoph is comparing it to the original md
    > >>architecture thich was more of an evolutionary design on the existing
    > >>block layer
    > >
    > >
    > > No, I do not. MD is in _no_ ways a volume managment framwork but just
    > > a few drivers that share common code. That's somethig entirely different.
    > So why then the requirement that internal remapping layers be
    > implemented as block devices?

    I don't care how a single remapping layers is implemented. I want
    the common Voulme managment API work on public nodes.

    > Neither is implementing an internal logical remapping layer as a
    > block device just so you can do an ioctl directly to it.

    Not without hacks.

    > I think the point is really explaining why they _should_ be accessed.
    > If there is some valid reason other than having something you
    > can do an ioctl on.

    Because that

    a) removes hacks like the EVMS pass-though
    b) allows userspace to easily access it through read/write

    > > argumentation tell me why you haven't submitted a patch to Linus
    > > yet to disallow direct access to block devices that are in use
    > > by a filesystem.
    > I think the issue here is an md block device in use by another md block
    > device. Possbily becuase md's design precludes this (a design artifact)
    > (ie. md tools need access to the intermediary devices - users don't).

    I'm not talksing about MD here. Why do you want to disallow people
    using a device just it has another layer above it. E.g. write a change
    to the ondisk structures (setting a flag, etcc..) is most logically
    expressed by simple, O_DIRECT write to the actual device.

    > Yes, but the block device encapsulation here removes the need for plugins
    > to be implemented as block devices ie. removing complexity elsewhere.
    > I must admit to not being an expert on the block layer - but wouldn't
    > your suggesed approach mean intermediary layers would each have a
    > request queue

    It _coukd_ have a request queue, yes.

    > and other unneeded stuff - if so, is this desirable?

    What unneeded stuff? block device state contains no state relevant
    to userspace access.

    > > This argument is NIL if the infrastructure is part of exactly that
    > > evolving block layer. You might have noticed that kernel code
    > > compatility to other releases is not really a criteria for the
    > > linux kernel development, btw..
    > I agree, maybe this would be worth doing for 2.7/2.8.


    > In the meatime
    > do you think this would be feasible? - you are basically suggesting
    > a complete rewrite


    > (or do you think you can do the rewrite to IBM's
    > satisfaction before the freeze ie. in the eternal linux kernel way,
    > you want it you write it ;). Me, i'm happy with the current approach
    > - but of course, i'm only a user ;).

    _I_ don't want to get EVMS in, sorry. I _do_ want a proper volume
    managment framework, but I can live with it not beeing in before 2.8.

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