Thanks for your answers!
On 2 dec 2011, at 02:54, Erik Trimble wrote:
> On 12/1/2011 4:59 PM, Ragnar Sundblad wrote:
>> 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?
> Yup. Each entry (not just a DDT entry, but any cached reference) in the L2ARC
> requires a pointer record in the ARC, so the DDT entries held in L2ARC also
> consume ARC space. It's a bad situation.
Yes, it is a bad situation. But how many DDT entries can there be in each ZAP
object? Some have suggested an 1:1 relationship, others have suggested that it
>> 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
>> 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
>> 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!
> Remember that, when using Dedup, each block can potentially be part of a very
> large number of files. So, when you delete a file, you have to go look at the
> DDT entry FOR EACH BLOCK IN THAT FILE, and make the appropriate DDT updates.
> It's essentially the same problem that erasing snapshots has - for each block
> you delete, you have to find and update the metadata for all the other files
> that share that block usage. Dedup and snapshot deletion share the same
> problem, it's just usually worse for dedup, since there's a much larger
> number of blocks that have to be updated.
What is it that must be updated in the DDT entries - a ref count?
And how does that differ from the snapshot case, which seems like
a very similar mechanism?
> The problem is that you really need to have the entire DDT in some form of
> high-speed random-access memory in order for things to be efficient. If you
> have to search the entire hard drive to get the proper DDT entry every time
> you delete a block, then your IOPs limits are going to get hammered hard.
>> 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.
> L2ARC typically sits on an SSD, and the DDT is usually held there, if the
> L2ARC device exists.
Well, it rather seems to be ZAP objects, referenced from the ARC, which
happens to contain DDT entries, that is in the L2ARC.
I mean that you could just move the entire AVL tree onto the SSD, completely
outside of zfs if you will, and have it being searched there, not dependent
of what is in RAM at all.
Every DDT lookup would take up to [tree depth] number of reads, but that could
be OK if you have a SSD which is fast on reading (which many are).
> There does need to be serious work on changing how the DDT in the L2ARC is
> referenced, however; the ARC memory requirements for DDT-in-L2ARC definitely
> need to be removed (which requires a non-trivial rearchitecting of dedup).
> There are some other changes that have to happen for Dedup to be really
> usable. Unfortunately, I can't see anyone around willing to do those changes,
> and my understanding of the code says that it is much more likely that we
> will simply remove and replace the entire dedup feature rather than trying to
> fix the existing design.
Yes, replacing it is certainly one possibility.
Is there any work going on for a replacement mechanism?
>> 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!
> bp_rewrite is a feature which stands for the (as yet unimplemented) system
> call of the same name, which does Block Pointer re-writing. That is, it would
> allow ZFS to change the physical location on media of an existing ZFS data
> slab. That is, bp_rewrite is necessary to allow ZFS to change the Physical
> layout of data on media, without changing the Conceptual arrangement of such
> It's been the #1 most-wanted feature of ZFS since I can remember, probably
> for 10 years now.
Yes, I got that much. :-)
But what is the problem really?
Being naive/ignorant (and completely ignoring any possible dependencies between
the different layers in the zfs stack), it doesn't seem that magic or esoteric
when compared to the rest of the stuff in there.
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