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    SubjectQuest: I/O prefetching experiment

    I am in the process of implementing an inter-file predictive
    prefetching policy. It uses accesses to one file to cause the
    prefetching of another file. (More details can be found at )

    My eventual goal is to produce and test a modified ext2 filesystem
    that has in each inode some information that indicates which
    inodes are related to this one, and then uses this information
    to pre-fetch those inodes and their data. Currently, I've
    created a module that sends traces of open and exec events to
    a device for reading and a user level deamon that reads these
    events -- builds the relationship structures and attempts
    to prefetch the inode and its data (through a gorry temporary hack in
    open -- I modified sys_open to call prefetching if the file name
    is of the format /#$=dev_numb#inode_numb).

    Ideally I'd like to check if the files data is alread in either the
    page or buffer, and if not set up an async I/O for the first 32 KB of this

    My questions:

    1 - Where is the best place to prefetch the regular file data to,
    the page cache and/or the buffer cache?

    My impression is that for regular file data the I should use the page
    cache, and for directory file data I should use the buffer cache.
    I am see cases where regular file data is found in the buffer
    cache but not in the page cache. How should this be happening
    if all regular file data reads go to pages in the page cache?

    2 - What is the best way to prefetch directory file information if I
    know that a specific directory is about to be accessed?



    Tom M. Kroeger Pray for wind
    Graduate Student, UC Santa Cruz \ Pray for waves
    e-mail: |\ and Pray it's your day off! |~\
    (408) 459-4458 |__\
    (408) 427-2001 home ,----+--

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