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    SubjectRe: Do ramdisk exec's map direct to buffer cache?
    Date
    In article <2947.963395714@cygnus.co.uk>,
    David Woodhouse <dwmw2@infradead.org> wrote:
    >
    >My current inclination is to make CONFIG_BLK_DEV a config option, turn it
    >off to save space, and use JFFS directly on flash chips, with ramfs for /tmp
    >and possibly also /var
    >
    >I'm fairly sure that ramfs does share pages as you'd hope it does.

    ramfs does indeed share pages, and does more than that: it shares the
    directory structure with the directory cache directly, so there is not
    any wasted memory even in meta-data. That was one of the design goals
    (along with extreme simplicity - it's the smallest and fastest
    filesystem around).

    That said, ramfs isn't perfect. It's not a "tmpfs" in that it cannot
    page anything out to disk (a non-issue in embedded devices, but it would
    make ramfs more useful in general use), and it's new code that isn't
    used by all that many people - so it could have (and has had) problems.
    As 99% of the functionality of ramfs is actually just using VFS code
    directly the basics of ramfs are very solid, but the devil is in the
    details..

    If you want to pre-populate ramfs (the way initrd does the old-style
    ramdisk), I would suggest using a compressed tar-file approach. In the
    long run I definitely want to get rid of the old-style ramdisk, as it
    has some serious problems both from a design standpoint and from a
    maintenance angle (the mm games it plays are much worse than the simple
    page-pinning stuff that ramfs can do - ramfs plays at a much higher
    level and has access to better abstractions through that).

    The only problem with JFFS is that it doesn't do compression, so there's
    a lot to be said for using cramfs if you have space constraints (and in
    embedded devices, if you don't have space constraints you're doing
    something wrong ;). So mixture of ramfs, cramfs and JFFS can be a good
    thing.

    Linus

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