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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