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From: Bruce Momjian <bruce@momjian.us>
Message-Id: <201005101549.o4AFnFQ20734@momjian.us>
Subject: Re: no universally correct setting for fsync
In-Reply-To: <AANLkTiktz9l00oZEjxvaMljYLebCnJQm-S9Vr4XVF071@mail.gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Date: Mon, 10 May 2010 11:49:15 -0400 (EDT)
CC: Michael Tharp <gxti@partiallystapled.com>,
	Craig Ringer <craig@postnewspapers.com.au>, pgsql-hackers@postgresql.org
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Robert Haas wrote:
> On Mon, May 10, 2010 at 11:12 AM, Bruce Momjian <bruce@momjian.us> wrote:
> > Michael Tharp wrote:
> >> On 05/08/2010 04:07 AM, Craig Ringer wrote:
> >> > It's probably worth mentioning that people who want to turn off fsync to
> >> > gain a performance boost should instead look at a RAID controller with a
> >> > BBU so they can safely enable write-back caching, getting most of the
> >> > benefits of fsync=off safely.
> >>
> >> Which options specifically should be set if a BBU is in use? Obviously
> >> fsync should be on always, but can full_page_writes be disabled? Are
> >> there other tweaks that can be done?
> >>
> >> It would be great to see some practical hints in the documentation while
> >> the fsync part is getting changed.
> >
> > Uh, our docs have:
> >
> > ? ? ? ?Turning this parameter off speeds normal operation, but might
> > ? ? ? ?lead to a corrupt database after an operating system crash or
> > ? ? ? ?power failure. The risks are similar to turning off
> > ? ? ? ?<varname>fsync</>, though smaller. ?It might be safe to turn
> > ? ? ? ?off this parameter 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 (e.g., ZFS).
> 
> "It might be safe" is a bit of a waffle.  It 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.

Agreed.  It is "safe" for us to be definitive here?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com