Philipp Matthias Hahn wrote:
I would also suggest one more feature: support for block level
de-duplication. I mean:
...
That would be very usable feature, which in most cases would allow to
shrink occupied disk space on 50-90%.
Have you references for this number?
In my experience one gets a lot of benefit from
the much simpler process of "de-duplication" of files.
Yes, I would expect simple hard links to be a better solution for this,
but the feature request is not that out of line. I actually had plans
on implementing auto duplicate block reuse earlier in btrfs.
One problem with hard-links for me is, they also share the meta-data,
especially file permissions and owners.
Take a Subversion checkout for example: For each file "$A" Subversion
saves a backup under ".svm/text-base/$A.svn-base" for file comparison
and diff generation. The user controls the file permissions of "$A",
Subversion protects its backup with 0444. You can't hard-link them,
because than "svn diff" doesn't work anymore if your editor doesn't
break the hard-link, or worse, your permissions can get wrong.
If previous versions Subversion also had an extra file for file
attributes (mime-type, permissions, to-be-ignored, etc.) Since most
files had no special attributes, each had a file only containing "END".
Those you could hard-link by hand to save space.
If somebody want to research this further:
There is this nice little package called "perforate", which contains
"finddup" to find duplicate files. Run it two times, once with "-i" to
ignore permissions while comparing file contents
finddup -i -d /
and once without "-i" for "content and permissions must match"
finddup -d /
This will give you a hint on how many files you could hard-link or how
many files share their content.
So, seems ever for file based de-duplication some support from the FS,
including some kind of ability for different inodes point to the same
data blocks to store the meta-data, would be needed anyway.
Vlad
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