linux-ext4 - Re: tune2fs: update journal super block when changing UUID for fs.

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Message-ID: <20140706024945.GA25768@thunk.org>
Date:	Sat, 5 Jul 2014 22:49:45 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Azat Khuzhin <a3at.mail@...il.com>
Cc:	linux-ext4@...r.kernel.org, chintzung@...il.com
Subject: Re: tune2fs: update journal super block when changing UUID for fs.

On Sat, Jun 28, 2014 at 06:29:46PM +0400, Azat Khuzhin wrote:
> 
> Here is the RFC patch for this, I'm not sure about the check for *is this 
> journal device or not*, so I will glad to see some comments. Thanks.

The way you tell whether or not you've opened the journal device is as
follows:

	if (fs->super->s_feature_incompat & EXT3_FEATURE_INCOMPAT_JOURNAL_DEV) {
	   ...

That is to say, the journal device has a ext2/3/4 superblock, but with
a special incompat flag which indicates that it is an external journal
--- and which also prevents the kernel from trying to mount the
external journal device as a normal ext4 file system.

Regards,

					- Ted
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