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Re: New XLOG record indicating WAL-skipping

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <gsstark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New XLOG record indicating WAL-skipping
Date: 2010-01-16 06:16:26
Message-ID: 3f0b79eb1001152216n1b7b572cj4104dd4b863a660e@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Jan 15, 2010 at 8:28 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> I don't like special-casing UNLOGGED records in XLogInsert and
> ReadRecord(). Those functions are complicated enough already. The
> special handling from XLogInsert() (and a few other places) is only
> required because the UNLOGGED records carry no payload. That's easy to
> avoid, just add some payload to them, doesn't matter what it is. And I
> don't think ReadRecord() is the right place to emit the errors/warnings,
> that belongs naturally in xlog_redo().
>
> It might be useful to add some information in the records telling why
> WAL-logging was skipped. It might turn out to be useful in debugging.
> That also conveniently adds payload to the records, to avoid the
> special-casing in XLogInsert() :-).
>
> I think it's a premature optimization to skip writing the records if
> we've written in the same session already. Especially with the 'reason'
> information added to the records, it's nice to have a record of each
> such operation. All operations that skip WAL-logging are heavy enough
> that an additional WAL record will make no difference. I can see that it
> was required to avoid the flooding from heap_insert(), but we can move
> the XLogSkipLogging() call from heap_insert() to heap_sync().
>
> Attached is an updated patch, doing the above. Am I missing anything?

Thanks a lot! Your change seems to be OK.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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