Message-ID: <4CC9DE0F.50401@mansionfamily.plus.com>
Date: Thu, 28 Oct 2010 21:33:19 +0100
From: James Mansion <james@mansionfamily.plus.com>
User-Agent: Thunderbird 2.0.0.24 (Windows/20100228)
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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>
In-Reply-To: <16724.1287952819@sss.pgh.pa.us>
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Tom Lane wrote:
> The other and probably worse problem is that there's no application
> control over how soon changes to mmap'd pages get to disk.  An msync
> will flush them out, but the kernel is free to write dirty pages sooner.
> So if they're depending for consistency on writes not happening until
> msync, it's broken by design.  (This is one of the big reasons we don't
> use mmap'd space for Postgres disk buffers.)
>   
Well, I agree that it sucks for the reason you give - but you use write 
and that's *exactly* the
same in terms of when it gets written, as when you update a byte on an 
mmap'd page.

And you're quite happy to use write.

The only difference is that its a lot more explicit where the point of 
'maybe its written and maybe
it isn't' occurs.

There need be no real difference in the architecture for one over the 
other: there does seem to be
evidence that write and read can have better forward-read and 
write-behind behaviour, because
read/write does allow you to initiate an IO with a hint to a size that 
exceeds a hardware page.

And yes, after getting into the details while starting to port TC to 
Windows, I decided to bin
it.  Especially handy that SQLite3 has WAL now.  (And one last dig - TC 
didn't even
have a checksum that would let you tell when it had been broken: but it 
might all be fixed now
of course, I don't have time to check.)

James