Message-ID: <4CCB340E.1000307@mansionfamily.plus.com>
Date: Fri, 29 Oct 2010 21:52:30 +0100
From: James Mansion <james@mansionfamily.plus.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Greg Smith <greg@2ndquadrant.com>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Bruce Momjian <bruce@momjian.us>, jd@commandprompt.com,
	Scott Marlowe <scott.marlowe@gmail.com>,
	Steve Crawford <scrawford@pinpointresearch.com>,
	pgsql-performance@postgresql.org, Ben Chobot <bench@silentmedia.com>
Subject: Re: BBU Cache vs. spindles
References: <4CC04EA50200002500036C5E@gw.wicourts.gov>
	<201010211935.o9LJZTM21744@momjian.us>
	<4CC0513E0200002500036C68@gw.wicourts.gov>
	<4CC3E8BF.4090409@mansionfamily.plus.com>
	<4CC46479.2050206@2ndquadrant.com> <16724.1287952819@sss.pgh.pa.us>
	<4CC9DE0F.50401@mansionfamily.plus.com>
	<26348.1288301177@sss.pgh.pa.us>
In-Reply-To: <26348.1288301177@sss.pgh.pa.us>
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Tom Lane wrote:
> Uh, no, it is not.  The difference is that we can update a byte in a
> shared buffer, and know that it *isn't* getting written out before we
>   
Well, I don't know where yu got the idea I was refering to that sort of 
thing - its
the same as writing to a buffer before copying to the mmap'd area.
> It's true that we don't know whether write() causes an immediate or
> delayed disk write, but we generally don't care that much.  What we do
>   
Which is what I was refering to.
> care about is being able to ensure that a WAL write happens before the
> data write, and with mmap we don't have control over that.
>
>   
I think you have just the same control either way, because you can only 
force ordering
with an appropriate explicit sync, and in the absence of such a sync all 
bets are off for
whether/when each disk page is written out, and if you can't ensure that 
the controller
and disk are write through you'd better do a hardware cache flush.too, 
right?

A shame that so many systems have relatively poor handling of that 
hardware flush.