linux-ext4 - Re: errors following ext3 to ext4 conversion

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Date:	Thu, 27 Aug 2015 14:58:45 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Chris Hunter <chris.hunter@...e.edu>,
	linux-ext4@...r.kernel.org
Subject: Re: errors following ext3 to ext4 conversion

On Wed, Aug 26, 2015 at 09:15:36PM -0700, Darrick J. Wong wrote:
> 
> tune2fs is not as strict as resize2fs; iirc resize whines if it finds ERROR
> status, lack of VALID status, or it having been too long since the last fsck,
> whereas tune2fs only cares that the fs is marked VALID.
> 
> (Scary, if you think about it...)

Originally tune2fs was for things like changing the number of reserved
blocks in the superblock, or setting a label, etc.  Things for which
subtle file system corruptions wouldn't be that big of a deal.

Even for setting feature flags, tune2fs doesn't make any fundamental
changes to the file system other than flipping a few bits.  So for
Chris, the good news is that undoing the tune2fs changes is relatively
easy if all he's done since then is to run a read-only e2fsck -n run.
We just have to flip a few bits.  (Note, the reason why I didn't
include ^dir_index is that most ext3 file systems created using
non-paleolithic versions of e2fsprogs will have dir_index turned on
already.)

But now that we have some tune2fs operations that do resize2fs-like
operations, we probably should add checks for those more risky
operations.  And even though feature-flags flipping isn't very scary
in and of itself, requiring maybe we should require it for that case
--- although we have historically supported adding things like the
extents flag, or even the journal when converting from ext2 to ext3,
while the file system was mounted.

I suspect that would fill Eric's heart with horror, but the ability to
migrate the root file system from ext2 to ext3 while it was mounted
(i.e., just run "tune2fs -O has_journal /dev/rootfs" and reboot) was
something Stephen Tweedie added, so at least at one point Red Hat was
more adventurous about what it would support in terms of file system
upgrades without using mkfs.  :-)

    	     		       	       - Ted
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