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Date: Sun, 19 Jul 2009 17:22:37 +0100
Message-ID: <9b2b86520907190922wb4f809dr5a315155211d6744@mail.gmail.com>
Subject: Re: disk Partition label changes and reflecting them in 
	/dev/disks-by-label/
From: Alan Jenkins <sourcejedi.lkml@googlemail.com>
To: Linda Walsh <lkml@tlinx.org>
Cc: LKML <linux-kernel@vger.kernel.org>
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On 7/19/09, Linda Walsh <lkml@tlinx.org> wrote:
>
>
> If I have a disk with no mounted partitions and I change the partition
> order,
> the OS will re-read the new partition and life goes on.
>
> However, if I create or change a new disk label, it seems label programs
> (and users) should have an option to reread the labels after the
> modification.
>
> Ideally any prog that changes a Label or UUID would trigger an update
> of what's in /dev/disks to reflect the new 'reality'.
>
> Currently, I go through through a seemingly bizarre ritual of invoking
> unmounting all other partitions on the same disk, then becoming root,
> running fdisk on the disk, then just exiting with "w".  This triggers
> a reread of not only the partition table, but also the new labels.
>
> However, I find this far less than ideal.  Is there a better way, or could
> there be a better way to update new Labels and UUID's that are actually
> on a disk -- perhaps even as an ordinary user command (since it would be
> a read-only operation on the disk that simply updates /dev/disk to reflect
> what's really there -- Especially being able to change only the label (or
> UUID), only on one partition w/o having to actually unmount other file
> systems
> on the disk....?
>
> Already implemented?  Or doable?  Or bad idea?
>
> Thanks,
> -linda

Already implemented :-).

The latest version of udev listens for inotify events on block
devices.  When a utility writes to the block device (or rather, when
it closes it after opening it for writing), Udev will synthesize a
"change" event.  The standard udev rules will re-check the device, and
update the links under /dev/disk to reflect changes in labels etc.

Alan
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