On 13.11.09 16:09, Ross wrote:
Isn't dedupe in some ways the antithesis of setting copies > 1? We go to a
lot of trouble to create redundancy (n-way mirroring, raidz-n, copies=n,
etc) to make things as robust as possible and then we reduce redundancy
with dedupe and compression
But are we reducing redundancy? I don't know the details of how dedupe is
implemented, but I'd have thought that if copies=2, you get 2 copies of each
dedupe block. So your data is just as safe since you haven't actually
changed the redundancy, it's just that like you say: you're risking more
data being lost in the event of a problem.
However, the flip side of that is that dedupe in many circumstances will free
up a lot of space, possibly enough to justify copies=3, or even 4.
It is not possible to set copies to 4. There's space for only 3 addresses in the
block pointer.
There's also dedupditto property which specifies a threshold, and if reference
count for deduped block goes above the threshold, another ditto copy of it is
stored automatically.
victor
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