> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Jim Klimov
> 2012-08-01 22:07, opensolarisisdeadlongliveopensolaris пишет:
> > L2ARC is a read cache.  Hence the "R" and "C" in "L2ARC."
> "R" is replacement, but what the hell ;)
> > This means two major things:
> > #1  Writes don't benefit,
> > and
> > #2  There's no way to load the whole DDT into the cache anyway.  So you're
> guaranteed to have performance degradation with the dedup.
> If the whole DDT does make it into the cache, or onto an SSD
> storing an extra copy of all pool metadata, then searching
> for a particular entry in DDT would be faster. When you write
> (or delete) and need to update the counters in DDT, or even
> ultimately remove an unreferenced entry, then you benefit on
> writes as well - you don't take as long to find DDT entries
> (or determine lack thereof) for the blocks you add or remove.
> Or did I get your answer wrong? ;)

Agreed, ARC/L2ARC help in finding the DDT, but whenever you've got a snapshot 
destroy (happens every 15 minutes) you've got a lot of entries you need to 
write.  Those are all scattered about the pool...  Even if you can find them 
fast, it's still a bear.
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