From: James Mansion <james@mansionfamily.plus.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Smith <greg@2ndquadrant.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: jd@commandprompt.com, Scott Marlowe <scott.marlowe@gmail.com>
Cc: Steve Crawford <scrawford@pinpointresearch.com>
Cc: pgsql-performance@postgresql.org, Ben Chobot <bench@silentmedia.com>
Subject: Re: BBU Cache vs. spindles
Date: Fri, 29 Oct 2010 21:52:30 +0100
Message-ID: <4CCB340E.1000307@mansionfamily.plus.com> (raw)
In-Reply-To: <26348.1288301177@sss.pgh.pa.us>
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>
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.
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To: pgsql-performance@postgresql.org
Cc: james@mansionfamily.plus.com, tgl@sss.pgh.pa.us, greg@2ndquadrant.com, Kevin.Grittner@wicourts.gov, bruce@momjian.us, scott.marlowe@gmail.com, scrawford@pinpointresearch.com, bench@silentmedia.com
Subject: Re: BBU Cache vs. spindles
In-Reply-To: <4CCB340E.1000307@mansionfamily.plus.com>
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