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From: Greg Stark <gsstark@mit.edu>
Date: Mon, 10 May 2010 18:46:53 +0100
Message-ID: <u2n407d949e1005101046weac382a6g99a5c932d4c5b736@mail.gmail.com>
Subject: Re: no universally correct setting for fsync
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>,
	Michael Tharp <gxti@partiallystapled.com>, pgsql-hackers@postgresql.org,
	Craig Ringer <craig@postnewspapers.com.au>
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On Mon, May 10, 2010 at 4:55 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>
>> "It might be safe" is a bit of a waffle. =A0It would be nice if we
>> could provide some more clear guidance as to whether it is or is
>> not, or how someone could go about testing their hardware to find
>> out.
>
> I think that the issue is that you could have corruption if some,
> but not all, disk sectors from a page were written from OS cache to
> controller cache when a failure occurred. =A0The window would be small
> for a RAM-to-RAM write, but it wouldn't be entirely *safe* unless
> there's some OS/driver environment where you could count on all the
> sectors making it or none of them making it for every single page.
> Does such an environment exist?

The reason for the waffle is that the following sentence describes a
whole set of environments based the following description:

> > ? ? ? ?if you have hardware (such as a battery-backed
> > ? ? ? ?disk controller) or file-system software that reduces the risk
> > ? ? ? ?of partial page writes to an acceptably low level

Depending on which set of hardware and how low the risk is it might be safe=
.

I think with WAFL or ZFS it's entirely safe. There may be other
filesystems with similar guarantees. With a BBU the risk might be very
low -- but it might not, it would be hard to determine without a
detailed analysis of the entire stack from the buffer cache,
filesystem, lvm, hardware drivers, BBU design, etc.

--=20
greg