linux-ext4 - Re: Mounting Ext3 with Ext4

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Message-ID: <baab8ccb0905202137v426c8568k8fc6f53fc095c351@mail.gmail.com>
Date:	Thu, 21 May 2009 00:37:11 -0400
From:	Matt LaPlante <cybrmatt@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Mounting Ext3 with Ext4

On Wed, May 20, 2009 at 9:28 PM, Matt LaPlante <cybrmatt@...il.com> wrote:
> On Wed, May 20, 2009 at 9:17 PM, Eric Sandeen <sandeen@...hat.com> wrote:
>> Matt LaPlante wrote:
>>> According to the Ext4 howto
>>> (http://ext4.wiki.kernel.org/index.php/Ext4_Howto#Converting_an_ext3_filesystem_to_ext4),
>>> "It is possible to mount both ext3 (and ext2, in kernels 2.6.28 and
>>> later) filesystems directly using the ext4 filesystem driver."  So
>>> this means, if I'm reading it correctly, that a kernel built with ext4
>>> support should be able to mount and manage ext[234] filesystems.  Is
>>> that correct?  If so, are there any limitations on this support?
>>>
>>> I'm running Ubuntu Jaunty with some custom-built 2.6.29.4 kernels and
>>> grub2.  I seem to have no problem booting from ext4 disks, so the ext4
>>> support is definitely there and functioning.  I had hoped the ext3
>>> support meant I could build the kernel with ext4 only and carry on as
>>> usual with my ext3 partitions.  Unfortunately doing so causes the ext3
>>> partitions to fail to boot.  So...
>>>
>>> booting ext3 with ext3 in the kernel=fine
>>> booting ext4 with ext4 in the kernel=fine
>>> booting ext3 with ext4 (only) in the kernel=not fine
>>>
>>> Is this a problem or was it never intended to work?
>>
>> It should work...
>>
>> It'd be more helpful if you could include details on exactly how it
>> failed, but if Ubuntu builds ext4 as a module, perhaps the initrd
>> creation sees root as ext3 and doesn't bother to put ext4 in the initrd?
>
> ext4 was compiled in directly, it was not a module (this wasn't an
> ubuntu kernel; it was stock).  I can try to get a stack trace with
> netconsole if it gets far enough to function.

It looks likes the magic touch was to add rootfstype=ext4 to the
kernel command line.  Prior to this the ext4 only kernel wasn't
finding the disk, but with it attached it now appears to boot
successfully.  Maybe worth adding a note to the wiki?

>
>>
>> -Eric
>>
>
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