Re: writing file to disk: not as easy as it looks

From: "Chris Friesen" <cfriesen@nortel.com>
To: Theodore Tso <tytso@mit.edu>
Cc: Pavel Machek <pavel@suse.cz>,
	mikulas@artax.karlin.mff.cuni.cz, clock@atrey.karlin.mff.cuni.cz,
	kernel list <linux-kernel@vger.kernel.org>,
	aviro@redhat.com
Subject: Re: writing file to disk: not as easy as it looks
Date: Tue, 02 Dec 2008 11:22:58 -0600	[thread overview]
Message-ID: <49356EF2.7060806@nortel.com> (raw)
In-Reply-To: <20081202163720.GB18162@mit.edu>

Theodore Tso wrote:

> Even for ext3/ext4 which is doing physical journalling, it's still the
> case that the journal commits first, and it's only later when the
> write happens that we write out the change.  If the disk fails some of
> the writes, it's possible to lose data, especially if the two blocks
> involved in the node split are far apart, and the write to the
> existing old btree block fails.

Yikes.  I was under the impression that once the journal hit the platter 
then the data were safe (barring media corruption).

It seems like the more I learn about filesystems, the more failure modes 
there are and the fewer guarantees can be made.  It's amazing that 
things work as well as they do...

Chris