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    Subject[RFC 0/7] [RFC] cramfs: fake write support
    Inspired by a discussion with Christoph Hellwig, I tried to
    recreate a patch that he did a few years ago to add support
    for writing to a mounted cramfs file system. It still has
    known problems (and likely unknown ones), but should be
    good enough for practical use. I've been able to boot
    a full Ubuntu installation from a cramfs image and work with
    it normally.

    The intention is to use it for instance on read-only root
    file systems like CD-ROM, or on compressed initrd images.
    In either case, no data is written back to the medium, but
    remains in the page/inode/dentry cache, like ramfs does.

    Many existing systems currently use unionfs or aufs for this
    purpose, by overlaying a tmpfs over a read-only file
    system like cramfs, squashfs or iso9660. IMHO, it would
    be a much nicer solution to not require unionfs for a simple
    case like this, but rather have support for it in the file
    system. If people find this useful, we can do the same in
    other read-only file system.

    Writing to existing files is broken in at least two corner
    cases, and I'm still looking for a solution here:

    When you truncate an on-disk to make it larger, reading
    beyond the old end of the file will make cramfs try to
    read from disk instead of filling with zeroes. I'm not sure
    if this can be solved without adding additional members to
    the inode structure (using a private inode cache) to remember
    the end of the on-disk file.

    Deleting a preexisting file currently does not free the inode
    and page cache for that file, which I assume is easy to fix.

    Also, the i_nlink field of directories is always 1, and
    has always been on cramfs. Getting the count right should
    simplify the code a bit and make it more correct according
    to posix, but will cost a bit of performance on 'stat'.

    The patch series also lives on
    git:// cramfs


    Arnd <><

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      Last update: 2008-05-31 17:43    [W:0.021 / U:31.540 seconds]
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