Re: dm-crypt using kthread
From: Jeff Garzik
Date: Sun Feb 15 2004 - 23:06:50 EST
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.
Jeff
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