> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Richard Elling
> I believe what you meant to say was "dedup with HDDs sux." If you had
> used fast SSDs instead of HDDs, you will find dedup to be quite fast.
>  -- richard

Yes, but this is a linear scale.  Suppose an SSD without dedup is 100x faster 
than a HDD without dedup.  And suppose dedup slows down a system by a factor of 
10x.  Now your SSD with dedup is only 10x faster than the HDD without dedup.  
So "quite fast" is a relative term.  The SSD with dedup is still faster than 
the HDD without dedup, but it's also slower than the SSD without dedup.

The extent of fibbing I'm doing is thusly:  In reality, an SSD is about equally 
fast with HDD for sequential operations, and about 100x faster for random IO.  
It just so happens that the dedup performance hit is almost purely random IO, 
so it's right in the sweet spot of what SSD's handle well.  You can't use an 
overly simplified linear model like I described above - In reality, there's a 
grain of truth in what Richard said, and also a grain of truth in what I said.  
The real truth is somewhere in between what he said and what I said.

No, the SSD will not perform as well with dedup as it does without dedup.  But 
the "suppose dedup slows down by 10x" that I described above is not accurate.  
Depending on what you're doing, dedup might slow down an HDD by 20x, and it 
might only slow down SSD by 4x doing the same work load.  Highly variable, and 
highly dependent on the specifics of your workload.

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