On Aug 2, 2012, at 5:40 PM, Nigel W wrote:

> On Thu, Aug 2, 2012 at 3:39 PM, Richard Elling <richard.ell...@gmail.com> 
> wrote:
>> On Aug 1, 2012, at 8:30 AM, Nigel W wrote:
>> Yes. +1
>> The L2ARC as is it currently implemented is not terribly useful for
>> storing the DDT in anyway because each DDT entry is 376 bytes but the
>> L2ARC reference is 176 bytes, so best case you get just over double
>> the DDT entries in the L2ARC as what you would get into the ARC but
>> then you have also have no ARC left for anything else :(.
>> You are making the assumption that each DDT table entry consumes one
>> metadata update. This is not the case. The DDT is implemented as an AVL
>> tree. As per other metadata in ZFS, the data is compressed. So you cannot
>> make a direct correlation between the DDT entry size and the affect on the
>> stored metadata on disk sectors.
>> -- richard
> It's compressed even when in the ARC?

That is a slightly odd question. The ARC contains ZFS blocks. DDT metadata is 
manipulated in memory as an AVL tree, so what you can see in the ARC is the
metadata blocks that were read and uncompressed from the pool or packaged
in blocks and written to the pool. Perhaps it is easier to think of them as 
in transition? :-)
 -- richard

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