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    SubjectRe: Question on FFS support
       Date:   Tue, 26 Oct 1999 15:24:25 -0500
    From: Brian Grayson <bgrayson@ibmoto.com>

    I don't know, but the original FreeBSD responder may have meant
    LFS, the log-structured filesystem, which the BSDs have
    recently revamped. It is a true journaling file system distinct
    from FFS/UFS, and not merely soft-updates on top of a
    traditional block-structured file system. With LFS, you always
    append on writes, rather than overwrite. Thus, writes are
    fast, and if the machine crashes, since the previous data has
    not been overwritten, you can recover quite quickly. Of
    course, with finite disks you need to do some
    garbage-collecting, but LFS does all of that for you, and also
    has some optimizations so that reads are still fast.

    The latter has been the traditional failing of LFS filesystems over
    update-in-place filesystems (i.e., FFS, and ext2) --- because you never
    write over an existing disk block, files and directories tend to get
    scattered all over the disk. There are ways things can be improved
    (intelligent log cleaners, and huge amounts of memory to cache data so
    you don't have to go disk for reads in the first place), but it's very
    difficult to make a log structured filesystem work as well as an
    update-in-place filesystem.

    There is a fairly widely held belief that LFS systems don't work well on
    smaller systems --- especially ones without a lot of memory --- because
    of the huge amounts of cache needed to make things reasonably fast. And
    there will be certain application access patterns which will be quite
    pessimal for LFS. There are systems where LFS works quite well, though.
    NetApp boxes use a log structured filesystem, for example. They've
    spent a lot of effort, using both hardware and software techniques, to
    make their boxes go *fast*.

    - Ted

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