Re: limiting hint bit I/O

2011/1/19 Robert Haas <robertmhaas(at)gmail(dot)com>:
> On Wed, Jan 19, 2011 at 11:52 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> ... So what we
>>> want to do is write a percentage of them, in a way that guarantees
>>> that they'll all eventually get written if people continue to access
>>> the same data.
>>
>> The word "guarantee" seems quite inappropriate here, since as far as I
>> can see this approach provides no such guarantee --- even after many
>> cycles you'd never be really certain all the bits were set.
>>
>> What I asked for upthread was that we continue to have some
>> deterministic, practical way to force all hint bits in a table to be
>> set.  This is not *remotely* responding to that request.  It's still not
>> deterministic, and even if it were, vacuuming a large table 20 times
>> isn't a very practical solution.
>
> I get the impression you haven't spent as much time reading my email
> as I spent writing it.  Perhaps I'm wrong, but in any case the code
> doesn't do what you're suggesting.  In the most recently posted
> version of this patch, which is v2, if VACUUM hits a page that is

Please update the commitfest with the accurate patch, there is only
the old immature v1 of the patch in it.
I was about reviewing it...

https://commitfest.postgresql.org/action/patch_view?id=500

> hint-bit-dirty, it always writes it.  Full stop.  The "20 times" bit
> applies to a SELECT * FROM table, which is a rather different case.
>
> As I write this, I realize that there is a small fly in the ointment
> here, which is that neither VACUUM nor SELECT force out all the pages
> they modify to disk.  So there is some small amount of remaining
> nondeterminism, even if you VACUUM, because VACUUM will leave the last
> few pages it dirties in shared_buffers, and whether those hint bits
> hit the disk will depend on a decision made at the time they're
> evicted, not at the time they were dirtied.  Possibly I could fix that
> by making SetBufferCommitInfoNeedsSave() set BM_DIRTY during vacuum
> and BM_HINT_BITS at other times.  That would nail the lid shut pretty
> tight.
>
> --
> Robert Haas
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>
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Cédric Villemain               2ndQuadrant
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