Re: Patch: Write Amplification Reduction Method (WARM)

On Sat, Feb 25, 2017 at 10:50 AM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
> On Fri, Feb 24, 2017 at 9:47 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> And there are no bugs, right? :-)
>
> Yeah yeah absolutely nothing. Just like any other feature committed to
> Postgres so far ;-)

Fair point, but I've already said why I think the stakes for this
particular feature are pretty high.

> I need to polish the chain conversion patch a bit and also add missing
> support for redo, hash indexes etc. Support for hash indexes will need
> overloading of ip_posid bits in the index tuple (since there are no free
> bits left in hash tuples). I plan to work on that next and submit a fully
> functional patch, hopefully before the commit-fest starts.
>
> (I have mentioned the idea of overloading ip_posid bits a few times now and
> haven't heard any objection so far. Well, that could either mean that nobody
> has read those emails seriously or there is general acceptance to that
> idea.. I am assuming latter :-))

I'm not sure about that. I'm not really sure I have an opinion on
that yet, without seeing the patch. The discussion upthread was a bit
vague:

"One idea is to free up 3 bits from ip_posid knowing that OffsetNumber
can never really need more than 13 bits with the other constraints in
place."

Not sure exactly what "the other constraints" are, exactly.

/me goes off, tries to figure it out.

If I'm reading the definition of MaxIndexTuplesPerPage correctly, it
thinks that the minimum number of bytes per index tuple is at least
16: I think sizeof(IndexTupleData) will be 8, so when you add 1 and
MAXALIGN, you get to 12, and then ItemIdData is another 4. So an 8k
page (2^13 bits) could have, on a platform with MAXIMUM_ALIGNOF == 4,
as many as 2^9 tuples. To store more than 2^13 tuples, we'd need a
block size > 128k, but it seems 32k is the most we support. So that
seems OK, if I haven't gotten confused about the logic.

I suppose the only other point of concern about stealing some bits
there is that it might make some operations a little more expensive,
because they've got to start masking out the high bits. But that's
*probably* negligible.

--
Robert Haas
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