linux-ext4 - Writing one file, observing two writes

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Date:	Sun, 13 Apr 2014 11:49:07 -0400
From:	Sebastian Biedermann 
	<biedermann@...eng.informatik.tu-darmstadt.de>
To:	linux-ext4@...r.kernel.org
Subject: Writing one file, observing two writes

Dear ext4 developers,

I'm working on a research project about side channel attacks to hard drives.
My testbed runs Ubuntu Linux 12.04 with ext4 file system.

When writing a random file with dd command to my hard drive, I could see
the process jbd2/sda1
writing two times while it seems to wait a fixed period of time between
these two writes.

I actually think that the process is first writing the data to the
sectors and then updating the ext4 data base in a second step.
Is that true or am I wrong? Why is it waiting some seconds between these
two steps and on what is that delay depending?

Thank you!
Sebastian
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