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Date: Fri, 14 Mar 2008 15:29:12 -0400
From: Theodore Tso <tytso@mit.edu>
To: Pavel Machek <pavel@ucw.cz>
Cc: Ric Wheeler <ric@emc.com>, Benny Amorsen <benny+usenet@amorsen.dk>,
       linux-kernel@vger.kernel.org
Subject: Re: writeback cache dangers Re: [ANNOUNCE] Ramback: faster than a
	speeding bullet
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On Fri, Mar 14, 2008 at 08:03:57PM +0100, Pavel Machek wrote:
> 
> > The ingest rate at the time of a power hit makes a huge 
> > difference as well - basically, pulling the power cord 
> > when a box is idle is normally not harmful. Try that 
> > when you are really pounding on the disks and you will 
> > see corruptions a plenty without barriers ;-)
> 
> I tried that, and could not get a corrruption. cp -a on big kernel
> trees, on sata disk with writeback cache and no barriers... and I
> could not cause fs corruption. ext3.
> 
> I'd like to demo danger of writeback cache. What should I do?

Ext3's journal probably hides a huge number of problems.  I'd try
something with a lot more parallel modifications to metadata.  Say
postmark with a large number of threads.  It would be interesting
actually to get some controlled results of exactly how busy a
filesystem has to be before you get filesystem corruption (which I
would check explicitly running "e2fsck -f" e2fsck after pulling the
plug on the drive).

					- Ted
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