linux-ext4 - Re: Feature request: e2fsck -z

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Message-ID: <alpine.LFD.2.00.1108101134580.4380@dhcp-27-109.brq.redhat.com>
Date:	Wed, 10 Aug 2011 11:35:34 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Ric Wheeler <ricwheeler@...il.com>
cc:	Andreas Dilger <adilger@...mcloud.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-ext4@...r.kernel.org
Subject: Re: Feature request: e2fsck -z

On Wed, 10 Aug 2011, Ric Wheeler wrote:

> On 08/10/2011 09:16 AM, Andreas Dilger wrote:
> > On 2011-08-10, at 12:34 AM, Ric Wheeler wrote:
> > > On 08/09/2011 06:52 PM, H. Peter Anvin wrote:
> > > > Hi all,
> > > > 
> > > > This is something I've wanted to see for a very long time, and it
> > > > finally occurred to me that perhaps I should say something about it!
> > > > 
> > > > It would be a very nice thing to have a flag to e2fsck, presumably -z,
> > > > to zero out any unused data blocks, inodes and so on.  The goal is to
> > > > minimize the amount of space required after compressing a virtual disk
> > > > image or similar, and to make sure any non-data isn't lying around.
> > > Do you need it to be in the fsck tool?
> > > 
> > > If you have a sparsely allocated block map under your file system, doing a
> > > zero of all blocks could add hours for a big, slow S-ATA drives (2-3 hours
> > > for a 1TB drive).
> > I think Ted has a tool that does this already.  It should be relatively
> > simple oo do, like "dd if=/dev/zero of=/mountpoint/temp_zero_file&&  rm
> > /mountpoint/temp_zero_file.
> 
> This will work but will be a potential multi-hour long process (and cause out
> of space errors at some point for other applications for a very brief window
> :))
> 
> > 
> > > An alternative for SSD's and devices that do TRIM/UNMAP would be to use
> > > one of the batched discard tools (that would make discarded data read back
> > > as zeroed).
> > In fact, I thought Lukas has already made a tool for sending BLKDISCARD for
> > all unused parts of the filesystem?
> > 
> > Cheers, Andreas
> 
> Right, I think he has. That tool (not part of fsck) would do the job much
> quicker for enabled devices,

You're right. It is a part of e2fsck and you can enable it with -E
discard.

Thanks!
-Lukas
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