linux-ext4 - Re: journal_dev + metadata

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Message-ID: <20140906071320.GC27293@birch.djwong.org>
Date:	Sat, 6 Sep 2014 00:13:20 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	TR Reardon <thomas_reardon@...mail.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: journal_dev + metadata_csum issues

On Thu, Sep 04, 2014 at 12:14:32AM -0400, TR Reardon wrote:
> 1) tune2fs hiccups when presented with journal device fs.  Should it instead
> at least report "this is a journal device" rather than "invalid super block"?

Yes.  e2fsck/debugfs seem to issue the 'unsupported features' complaint but
without the 'invalid superblock' wording.

> 2) no way to create journal_dev with metadata_csum.  This would provide
> checksum for the fs superblock.

Not sure how useful this is since most of the SB is irrelevant here.  But I
don't see any reason why we shouldn't let users turn it on.

> 3) dumpe2fs should still display journal flags for journal_dev; currently it
> fails to display journal flags.

Ick.  Yes, that should work.

> 4) s_jnl_blocks in the superblock should be zeroed when removing a journal
> (ie ^has_journal) or when setting the journal to journal_dev.  Currently, the
> legacy (now dead) block list is maintained.  I'd argue that will invite
> misuse.

Seems like a reasonable precaution.

Now, does anyone know why ext4 reports a df size of 64ZB when I create an
external journal FS? :)

--D
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