[linux-lvm] Data deduplication for Linux : lessfs
Les Mikesell
lesmikesell at gmail.com
Wed Jun 24 21:21:59 UTC 2009
malahal at 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 at gmail.com
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