linux-kernel - Re: A Plumber’s Wish List for Linux

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Date:	Fri, 7 Oct 2011 16:21:37 +0100
From:	Hugo Mills <hugo-lkml@...fax.org.uk>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	lennart@...ttering.net, harald@...hat.com, david@...ar.dk,
	greg@...ah.com, Chris Mason <chris.mason@...cle.com>,
	Btrfs mailing list <linux-btrfs@...r.kernel.org>
Subject: Re: A Plumber’s Wish List for Linux

On Fri, Oct 07, 2011 at 02:46:23PM +0200, Kay Sievers wrote:
> On Fri, Oct 7, 2011 at 12:38, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > On Fri, 7 Oct 2011 12:28:46 +0200 Kay Sievers <kay.sievers@...y.org> wrote:
> >
> >> What do you mean would be ugly?
> >
> > I have an ext4fs. It supports every possible file name allowed by POSIX
> > and SuS. What name are you going to use for your 'hidden directory' that
> > won't clash with a real file ?
> 
> Ah, no. The label on FAT (similar on NTFS) are 'magic entries' in the
> root dir list, not a real file in the root dir.
> 
> We need kernel support for changing a mounted fs, because, unlike
> ext4, the blocks containing the strings are inside the fs, which the
> kernel might change any time.

   It's worth noting that there are similar issues with btrfs around
changing label. A common API for it would make sense. The only btrfs
patches I've seen to change label after mkfs-time work either as:

 * unmounted only, single underlying device only, pure userspace
   implementation
 * mounted only, multiple underlying devices, kernel support needed

   The kernel-side patches never got integrated, so we're still unable
to change the label on the majority of btrfs filesystems.

   Changing the UUID for the filesystem is even harder, as I think
it's written to every metadata block. I'm not sure we can do that
sanely on a mounted filesystem.

   Hugo (just a spear-carrier from the btrfs chorus).

-- 
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