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    SubjectRe: dm-crypt using kthread
    Grzegorz Kulewski wrote:
    > Could somebody write dm-compress (compressing not encrypting)? Is it
    > technically possible (can device mapper handle different data size at
    > input, differet at output)? (I think there is compressing loop patch.)
    > Could dm first compress data (even with weak algorithm), then encrypt, to
    > make statistical analysis harder?

    It's certainly possible, but you have to consider that data transfer
    almost always should be considered in page-sized chunks. For compress
    that would imply you would need to allocate/free blocks and similar
    duties that a filesystem must perform, simply because you do not have
    one-to-one correspondence with blocks being passed to you.

    You also have to consider that the kernel may request one or more pages
    that are in the middle of a compressed run of pages. For example,
    consider an algorithm that compresses 16 pages into a run of 4 pages.
    Later on, when the kernel requests (uncompressed) page 9, you likely
    need to read all 4 pages, and allocate 16 more pages for decompression.
    So, reading 1 upper layer page required dm-compress tying up 20 pages.


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