linux-kernel - Ext4 and the "30 second window of death"

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Message-Id: <200903291224.21380.info@gnebu.es>
Date:	Sun, 29 Mar 2009 12:24:21 +0200
From:	Alberto Gonzalez <info@...bu.es>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Ext4 and the "30 second window of death"

Hi,

Reading this discussion about the fsync performance problems, the reliability 
of delayed allocation, etc... made me a bit confused, so as a normal user I 
would like to ask a clear question with an example so I can get a clear answer 
and understand the implications of all this.

- Let's say I'm a writer and I like to take my laptop to a cafe every day to 
write there for a few hours.

- As such, I want to get good battery life so I'm fine with my data being 
written to death say every 30 seconds instead of waking up the disk 
immediately if I save the document I'm working on.

- I use Ext4 as my filesystem (default in next Fedora release).

- Let's say I've been working on my book for the last 14 months and I've 
written about 400 pages on an ODF file.

- My usual workflow is that every time I finish a paragraph, say every 2-3 
minutes, I hit Ctrl+S to save the changes.

- So one day, while I'm working on the book the following happens: I finish a 
paragraph and his Ctrl+S to save it. 5 seconds later the system freezes for 
some reason. Let's suppose that in that 5 window timeframe between pressing 
Ctrl+S and the crash the data has not been written to disk (which happens 
every 30 seconds). So as a result I:

A - Lose that last paragraph
B - Lose the whole book

If it's 'A', then that's ok, as expected. Bad luck. But if it's 'B', then I 
think that's totally unexpected by any user, and totally unacceptable too. 
Sure I want good performance and good battery life, but not at such cost. 
(Yes, you can argue I should have a recent backup at home, and you'd be right, 
but that doesn't change things fundamentally).

As far as I understand, with Ext3 (defaults), the behavior was A. Will this 
change to B with Ext4 and all "modern" filesystems (XFS, Btrfs,...)?

Thanks for any answer.
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