> Once something is written deduped you will always use the memory when
> you want to read any files that were written when dedup was enabled, so
> you do not save any memory unless you do not normally access most of
> your data.

For reads you don't need ddt. Also in Solaris 11 (not in Illumos
unfortunately AFAIK) on reads the in-memory ARC will also stay deduped (so
if 10x logical blocks are deduped to 1 and you read all 10 logical copies,
only one block in arc will be allocated). If there are no further
modifications and you only read dedupped data, apart from disk space
savings, there can be very nice improvement in performance as well (less
i/o, more ram for caching, etc.).

> As far as the OP is concerned, unless you have a dataset that will
> dedup well don't bother with it, use compression instead (don't use
> both compression and dedup because you will shrink the average record
> size and balloon the memory usage).

Can you expand a little bit more here?
Dedup+compression works pretty well actually (not counting "standard"
problems with current dedup - compression or not).

Robert Milkowski

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