On Tue, Aug 2, 2016 at 3:18 PM, Thomas Caputi <[email protected]> wrote:
> I sent these questions to Matt Ahrens, but it occurred to me later
> that this might be a better place to ask.
Guess I'll copy my response here then.
1) What does the ddt_phys_type_t enum represent and what do the values
> mean? Specifically, what is the difference between DDT_PHYS_DITTO and
> the others?
>
In the value of the DDE (ddt_phys_t), it can have up to 4 BP's, one for
each number of DVA's (single, double triple), plus one additional BP for
the extra "dedup ditto" copy. The zpool manpage describes this:
dedupditto=number
Threshold for the number of block ditto copies. If the reference
count for a deduplicated block increases above this number, a new
ditto copy of this block is automatically stored. The default
setting is 0 which causes no ditto copies to be created for
deduplicated blocks. The miniumum legal nonzero setting is 100.
>
> 2) What does the ddt_class_t enum represent and what do the values
> mean? Why do we keep different dedup objects on disk and in-core for
> each class in this enum? This seems related to question 1.
>
The DDE's are separated out into 3 different "classes" each of which is
stored in its own ZAP object. The reason for this is probably some alleged
performance benefit which was never well-measured. The classes are:
DITTO - DDE's that have a "dedup ditto" copy, i.e. their refcount is > the
dedupditto property
DUPLICATE - DDE's that have refcount > 1 but < dedupditto
UNIQUE - DDE's with refcount == 1
>
> 3) It looks to me that each ddt_phys_t can support up to
> SPA_DVAS_PER_BP (3) DVAs and each ddt_entry_t can support up to
> DDT_PHYS_TYPES (4). Does that mean that the dedup table can store up
> to 12 copies of each dedup'd block? That seems a bit excessive.
>
Theoretically, yes, but that doesn't happen in practice. Practically it
could be 1+2+3+1=7 (SINGLE + DOUBLE + TRIPLE + DITTO) DVAs.
--matt
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