linux-ext4 - Re: Ext3 behavior on power failure

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Message-ID: <20070328132903.GI14935@atrey.karlin.mff.cuni.cz>
Date:	Wed, 28 Mar 2007 15:29:04 +0200
From:	Jan Kara <jack@...e.cz>
To:	"John Anthony Kazos Jr." <jakj@...-k-j.com>
Cc:	Ric Wheeler <ric@....com>, armangau_philippe@....com,
	ext3-users@...hat.com, linux-ext4@...r.kernel.org,
	csar@...nford.edu
Subject: Re: Ext3 behavior on power failure

> > If you fsync() your data, you are guaranteed that also your data are
> >safely on disk when fsync returns. So what is the question here?
> Pardon a newbie's intrusion, but I do know this isn't true. There is a 
> window of possible loss because of the multitude of layers of caching, 
> especially within the drive itself. Unless there is a super_duper_fsync() 
> that is able to actually poll the hardware and get a confirmation that the 
> internal buffers are purged?
  OK :), to correct myself: After fsync() returns, all the data is acked from
the disk (or at least it should be like that unless there's a bug
somewhere). So if there are some caches in the hardware which the hardware
is not able to flush on power failure, that's a bad luck... That's why
you should turn off write caching on cheaper disks if you really care
about data integrity.

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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