I am sorry if these are dumb questions. If there are explanations
available somewhere for those questions that I just haven't found, please
let me know! :-)
1. It has been said that when the DDT entries, some 376 bytes or so, are
rolled out on L2ARC, there still is some 170 bytes in the ARC to reference
them (or rather the ZAP objects I believe). In some places it sounds like
those 170 bytes refers to ZAP objects that contain several DDT entries.
In other cases it sounds like for each DDT entry in the L2ARC there must
be one 170 byte reference in the ARC. What is the story here really?
2. Deletion with dedup enabled is a lot heavier for some reason that I don't
understand. It is said that the DDT entries have to be updated for each
deleted reference to that block. Since zfs already have a mechanism for sharing
blocks (for example with snapshots), I don't understand why the DDT has to
contain any more block references at all, or why deletion should be much harder
just because there are checksums (DDT entries) tied to those blocks, and even
if they have to, why it would be much harder than the other block reference
mechanism. If anyone could explain this (or give me a pointer to an
explanation), I'd be very happy!
3. I, as many others, would of course like to be able to have very large
datasets deduped without having to have enormous amounts of RAM.
Since the DDT is a AVL tree, couldn't just that entire tree be cached on
for example a SSD and be searched there without necessarily having to store
anything of it in RAM? That would probably require some changes to the DDT
lookup code, and some mechanism to gather the tree to be able to lift it
over to the SSD cache, and some other stuff, but still that sounds - with
my very basic (non-)understanding of zfs - like a not to overwhelming change.
4. Now and then people mention that the problem with bp_rewrite has been
explained, on this very mailing list I believe, but I haven't found that
explanation. Could someone please give me a pointer to that description
(or perhaps explain it again :-) )?
Thanks for any enlightenment!
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