linux-ext4 - Re: zero out blocks of freed user data for operation a virtual machine environment

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Date:	Sun, 24 May 2009 10:15:51 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Thomas Glanzmann <thomas@...nzmann.de>
Cc:	tytso@...nk.org, LKML <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: zero out blocks of freed user data for operation a virtual
 machine environment

On Sun, 24 May 2009 19:00:45 +0200
Thomas Glanzmann <thomas@...nzmann.de> wrote:

> Hello Ted,
> I would like to know if there is already a mount option or feature in
> ext3/ext4 that automatically overwrites freed blocks with zeros? If
> this is not the case I would like to know if you would consider a
> patch for upstream? I'm asking this because I currently do some
> research work on data deduplication in virtual machine environments
> and corresponding backups. It would be a huge space saver if there is
> such a feature because todays and tomorrows backup tools for virtual
> machine environments work on the block layer (VMware Consolidated
> Backup, VMware Data Recovery, and NetApp Snapshots). This is not only
> true for backup tools but also for running Virtual machines. The case
> that this future addresses is the following: A huge file is
> downloaded and later delted. The backup and datadeduplication that is
> operating on the block level can't identify the block as unused. This
> results in backing up the amount of the data that was previously
> allocated by the file and as such introduces an performance overhead.
> If you're interested in real live data, I'm able to provide them.
> 
> If you don't intend to have such an optional feature in ext3/ext4 I
> would like to know if you know a tool that makes it possible to zero
> out unused blocks?
> 

wouldn't it be better if the VM's would just support the TRIM command?


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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