Remember my recent disk issues?  As a side effect of protecting myself, I have a good example of deduplication results.

I have a second disk in my server, with slave Hammer PFSs to match what’s on my main disk.  I hadn’t put them in fstab, so they weren’t getting mounted and updated.  I got them re-created, but they were nearly full.  Here’s an abbreviated df, from which you should be able to tell which drives I have :

Size   Used   Avail   Capacity
929G   729G   200G    78%    /slave/slavehome
929G   729G   200G    78%    /slave/slavevar
929G   729G   200G    78%    /slave/slaveusr
929G   729G   200G    78%    /slave/slaveslash

That 78% is how full the Hammer volume was.  I turned on Hammer deduplication, since it’s off by default.  The very next day:

Size   Used   Avail   Capacity
929G   612G   318G    66%    /slave/slavehome 
929G   612G   318G    66%    /slave/slavevar 
929G   612G   318G    66%    /slave/slaveusr 
929G   612G   318G    66%    /slave/slaveslash

It’s a 1 terabyte disk, and I gained more than 10% back – That’s 100g of disk space that I gained overnight.  There might be more tomorrow, given that it was all of 5 minutes of dedup work.

This won’t surprise you if you’ve seen previous deduplication links here, like my previous results or some real-world tests.  It’s still great.  I’d suggest turning it on if you haven’t – hammer viconfig the appropriate PFS and uncomment the dedup line.