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    SubjectRe: Transparent compression in the FS
    Christopher Li wrote:
    >>The idea of this sort of block level hashing to allow
    >>sharing of identical blocks seems attractive but i wouldn't
    >>trust any design that did not accept as given that there
    >>would be false positives. This means that a write would
    >>have to not only hash the block but then if there is a
    >>collision do a compare of the raw data. Then you have to
    >>add the overhead of having lists of blocks that match a hash
    >>value and reference counts for each block itself. Further,
    >
    >
    > Then write every data block will need to dirty at least 2 blocks.
    > And it also need to read back the original block if hash exist.
    > There must be some performance hit.


    In my case at least, we're talking about archival storage. Plan9 uses a
    "write buffer" of 1-2GB or so, to mitigate performance loss, which seems
    reasonable. With archival storage and hash indexes and such, you're
    certainly going to be dirtying more disk blocks than a traditional local
    filesystem would.

    Jeff



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