linux-ext4 - Re: [PATCH] e2fsck: Discard free data and inode blocks.

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Message-ID: <alpine.LFD.2.00.1010222029100.3007@dhcp-lab-213.englab.brq.redhat.com>
Date:	Fri, 22 Oct 2010 20:31:47 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Lukas Czerner <lczerner@...hat.com>,
	Ric Wheeler <ricwheeler@...il.com>, linux-ext4@...r.kernel.org,
	tytso@....edu
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

On Fri, 22 Oct 2010, Eric Sandeen wrote:

> Andreas Dilger wrote:
> > On 2010-10-22, at 08:32, Lukas Czerner wrote:
> >>>> There is a concern that discard might prevent data recovery
> >>>> after fsck because it might be already discarded (some weird fs
> >>>> corruption?) in pass 5. However in my opinion this is a very
> >>>> small window (if there even is any), because we have already
> >>>> passed check 1-4 and we have just confirmed that group
> >>>> descriptors should be ok.
> > 
> > I don't totally agree.  When users have a serious filesystem problem,
> > the first thing they normally do is run e2fsck to see if it is
> > corrected (it may even be done automatically at boot after
> > errors=panic causing a reboot.
> > 
> > After that, they may want to recover some more data (e.g. with
> > ext3grep, or restore an e2image of the metadata, and re-run e2fsck).
> > If e2fsck will discard all of the data then any data recovery will be
> > impossible.
> 
> Could set it to only issue discard when the check was clean....
> 
> It could still be an option of course, but that might be safer still.
> 
> -Eric
> 

Well, this is the way I am doing it now: in pass 5 (which is the last
one), after the group descriptors has been checked, so there is an
assumption that it should be clean. But I agree, that it should be off
by default in fsck.

-Lukas
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