Disk usage and ZFS deduplication

[jhell]

On Tue, Jun 14, 2011 at 09:19:32AM +0200, Per von Zweigbergk wrote:
> I've been following the "Impossible compression ratio on ZFS" thread with some interest, and it made me ask myself this:
> 
> Let us say we have a hypothetical zfs filesystem with the equally hypothetical files A and B. The filesystem has deduplication enabled. Both files have an apparent file size of 100 MB, but 50 MB of that data is common between the two files and thus can be deduplicated. This would mean that total disk usage would be 150 MB.
> 
> If you use "du" to determine disk size for a deduplication, what would be the result? Which file would the common data be accounted to? Or would it be accounted to both files somehow, in part or in full?

Logical answer would be that both files should be showing thier
resulting size regardless of how ZFS processes them. Being deduped does
not mean representing files to the user any different.
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