Colin,
I think you mix up the filesystem layer (where the individual files as
maintained) and the block layer, where actual data is stored.
The analogue of deduplication on the filesystem layer would be to create hard
links of the files, where deleting one file does not remove the other link.
Block layer deduplication is a black box, see it simply as a compression,
which works in the background.
Thomas
* Colin Raven [Tue Dec 08, 2009 at 01:00:54PM +0100]:
In reading this blog post:
[1]http://blogs.sun.com/bobn/entry/taking_zfs_deduplication_for_a
a question came to mind.....
To understand the context of the question, consider the opening paragraph
from the above post;
Here is my test case: I have 2 directories of photos, totaling about
90MB each. And here's the trick - they are almost complete duplicates of
each other. I downloaded all of the photos from the same camera on 2
different days. How many of you do that ? Yeah, me too.�
OK, I consider myself in that category most certainly. Through just plain
'ol sloppiness I must have multiple copies of some images. Sad self
indictment...but anyway....
What happens if, once dedup is on, I (or someone else with delete rights)
open a photo management app containing that collection, and start deleting
dupes - AND - happen to delete the original that all other references are
pointing to. I know, I know, it doesn't matter - snapshots save the day -
but in this instance that's not the point because I'm trying to properly
understand the underlying dedup concept.
Logically, if you delete what everything is pointing at, all the pointers
are now null values, they are - in effect - pointing at nothing...an empty
hole.
I have the feeling the answer to this is; "no they don't, there is no
spoon ("original") you're still OK". I suspect that, only because the
people who thought this up couldn't possibly have missed such an "obvious"
point. The problem I have is in trying to mentally frame this in such a
way that I can subsequently explain it, if asked to do so (which I see
coming for sure).
Help in understanding this would be hugely helpful - anyone?
Regards & TIA,
-Me
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