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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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