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    SubjectRe: embedded flash filesystems
    On Fri, 14 Jul 2000, Jim Gettys wrote:
    > We suspect that a combination of cramfs and jffs will serve handhelds
    > very well... I can say from first hand experience that compressed ramdisks
    > and/or cramfs gets very old, very quickly: we really want a writable file
    > system, with (read) compression.

    Ok, that sounds like a useful embedded setup. Notice that when we've added
    compression and made an improvement to the flash-sector allocator, you
    could actually use JFFS only (with write-compression disabled) which would
    provide the read-only files as pre-compressed (through the mkfs.jffs tool)
    and stationary in their sectors, while handling the writable files into
    the rest of the sectors. I guess enabling write-compression could be made
    with a mount-flag.

    (Right now the GC is a bit stupid and moves around nodes that don't really
    change, so it's suboptimal to use it on a single partition with like 90%
    read-only data..)

    > A scheme Keith Packard proposed which would work very well for read only
    > data is somewhat similar to cramfs: just compress on 4K boundaries
    > and store an offset table, and mark the file as compressed when you are
    > done; do the obvious uncompression on the marked files on read. My intuition
    > is that this might be a performance win even on vanilla machines today.

    The compression in JFFS will work similarly - nodes will be created with a
    max-size limit (probably smaller using compression than without) and the
    read_page routine will just uncompress the enclosing node(s) and give out
    the data (in the special case of max-node-size == PAGE_SIZE it will be
    especially simple of course (like in cramfs) and maybe that provides good
    enough compression).


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