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    SubjectRe: [patch] pagecache-2.3.9-H3, bmap & ext2fs cleanup patch

    On Sun, 27 Jun 1999, Ingo Molnar wrote:
    > no, RAID is really a different concept, and it's filesystem independent.
    > multi-device filesystems _might_ be useful but the cleanest way of
    > handling striping and/or redundancy is IMO MD at the block device layer.
    > Caching and block allocation decisions are best done on the filesystem
    > level.

    The best argument for having the filesystem know about devices is to have
    filesystems that can "migrate" files from one device to another. It can be
    done other ways too, but basically it is really nice for "intelligent"
    filesystems that do (for example) dynamic load balancing or that have
    multiple levels of caching.

    Basically I'm talking about hierarchical filesystems: everything looks
    like one large filesystem, but the filesystem itself can dynamically move
    things from a slow medium to a fast medium according to usage patterns.
    Old stuff that is seldom used is on a slow disk (or disk arrays), while
    stuff that is used regularly is on a fast one.

    I'm not writing one, and I don't know of anybody _else_ writing one
    either, but this kind of schenario is why I think the VFS layer should not
    ever assume that we have just one device. So when we're cleaning up other
    assumptions, we're better off cleaning this one up too - especially as
    it's so simple and so "obvious" (the "Mapped" attribute really means that
    both the block number and the device is known, so it makes much more
    conceptual sense to have the mapper do both).


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