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    SubjectRe: Linux Buffer Cache Does Not Support Mirroring

    Linus,

    I can get done what I need for ship of the full NWFS (with mirroring) if
    I can get an async interface to the disks that isn't tied into the
    buffer cache from a memory consumption viewpoint. At present, the
    interface still requires that I call getblk() for any buffer heads I
    want to pass to the drivers. I need a way not to have to do this, so I
    could simply pass my own buffer_head elements into ll_rw_block or
    make_request(). I've found that when I do, all the drivers
    malfunction. I want very much to work within a "Linux" framework, but I
    realize that what's there just aint gonna' work without a sledge hammer
    approach. Can you help?

    Please advise,

    Jeff

    Linus Torvalds wrote:
    >
    > In article <381DC5A0.CABB7735@timpanogas.com>,
    > Jeff V. Merkey <jmerkey@timpanogas.com> wrote:
    > >
    > >What everyone is side-stepping is that the interface between the drivers
    > >and the buffer cache is incestuous -- this prevents folks from building
    > >async I/O based FS's on Linux. The solution is not a simple one -- the
    > >drivers and buffer cache interface needs to be changed to elimnate these
    > >dependencies so.
    >
    > Actually, not at all.
    >
    > The way to fix what you call "incestuous" is to just marry the two off
    > for good. They aren't incestuous, they're just living in sin..
    >
    > The buffer cache is closely related to the drivers, and will become only
    > _MORE_ closely related to the drivers. That's because the buffer cache
    > is quite consciously being evolved into just a driver interface for IO:
    > the "buffer cache" is transforming away from a "cache", and into a pure
    > "block IO interface".
    >
    > So no, the buffer cache doesn't support the kind of mirroring you want,
    > and almost certainly never will. But the page cache may eventually
    > evolve into the direction you're looking for.
    >
    > Linus
    >
    > -
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