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    SubjectRe: [PATCH 1/5] Forking ext4 filesystem from ext3 filesystem
    On Thu, 10 Aug 2006 16:13:33 -0400
    Jeff Garzik <jeff@garzik.org> wrote:

    > The sooner we kill buffer heads and use submit_bio(), the better :)

    A buffer_head is a caching entity and a bio is an IO container. They're
    quite separate concepts.

    A buffer_head is the kernel's sole abstraction of a disk block.
    Filesystems use disk blocks a lot, and they need such an abstraction.

    If one was to replace buffer_heads with direct-to-BIO operations then the
    filesytem would need to internally track the mapping from

    page+offset+length -> disk block

    and it would need to internally track the page+offset+length<->disk block
    coherency state and it would need to internally perform serialisation of
    access to each page+offset+length hunk of pagecache and etc and etc and
    etc. Create a data structure with which to do all that and voila,
    buffer_heads reinvented.
    -
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