Re: zero out blocks of freed user data for operation a virtualmachine environment

From: Arjan van de Ven
Date: Sun May 24 2009 - 13:16:08 EST

On Sun, 24 May 2009 19:00:45 +0200
Thomas Glanzmann <thomas@xxxxxxxxxxxx> 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
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