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    SubjectRe: [RFC][PATCH] ChunkFS: fs fission for faster fsck
    On Tue, 24 Apr 2007, Nikita Danilov wrote:

    > Amit Gud writes:
    >
    > Hello,
    >
    > >
    > > This is an initial implementation of ChunkFS technique, briefly discussed
    > > at: http://lwn.net/Articles/190222 and
    > > http://cis.ksu.edu/~gud/docs/chunkfs-hotdep-val-arjan-gud-zach.pdf
    >
    > I have a couple of questions about chunkfs repair process.
    >
    > First, as I understand it, each continuation inode is a sparse file,
    > mapping some subset of logical file blocks into block numbers. Then it
    > seems, that during "final phase" fsck has to check that these partial
    > mappings are consistent, for example, that no two different continuation
    > inodes for a given file contain a block number for the same offset. This
    > check requires scan of all chunks (rather than of only "active during
    > crash"), which seems to return us back to the scalability problem
    > chunkfs tries to address.

    not quite.

    this checking is a O(n^2) or worse problem, and it can eat a lot of memory in
    the process. with chunkfs you divide the problem by a large constant (100 or
    more) for the checks of individual chunks. after those are done then the final
    pass checking the cross-chunk links doesn't have to keep track of everything, it
    only needs to check those links and what they point to

    any ability to mark a filesystem as 'clean' and then not have to check it on
    reboot is a bonus on top of this.

    David Lang

    > Second, it is not clear how, under assumption of bugs in the file system
    > code (which paper makes at the very beginning), fsck can limit itself
    > only to the chunks that were active at the moment of crash.
    >
    > [...]
    >
    > >
    > > Best,
    > > AG
    >
    > Nikita.
    > -
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