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Re: [linux-lvm] Data deduplication for Linux : lessfs



malahal us ibm com wrote:

Block level deduplication isn't going to know/care about the difference between file contents and metadata. It is either stored in blocks that match other blocks or not and the difference should not be visible to the filesystem living on top of the block device.
My point exactly. If dedup was to be done on the block layer, you'd need flag to say "do not dedup this".
Why? How can it possibly make any difference? It's not likely that you'd have dupes in the metadata block, but if you do it doesn't matter that they are transparently mapped into one. You need a copy-on-write mechanism anyway since if you write to either they won't be dups any more.

Because some file systems create duplicate copies of metadata for
recovery if there is some sectors go bad on the media. You really don't
want to merge them!

My experience with disks is that if any part of them fails you don't want to trust data from any other part. So I'd consider this a big waste of time and generally keep data that matters on mirrored drives. Hmmm, I suppose you would want it to know not to de-dup the mirrored blocks..

--
  Les Mikesell
   lesmikesell gmail com


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