linux-kernel - Re: Please add generic support for root=UUID= at kernel parameter command line (LABEL, BYID maybe also)

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Message-ID: <4C72EE8F.3040708@turmel.org>
Date:	Mon, 23 Aug 2010 17:56:31 -0400
From:	Phil Turmel <philip@...mel.org>
To:	Eric.Valette@...e.fr
CC:	linux-kernel@...r.kernel.org
Subject: Re: Please add generic support for root=UUID= at kernel parameter
 command line (LABEL, BYID maybe also)

(Adding cut CC...  sorry about the duplicate)

Hi Eric,

On 08/23/2010 10:37 AM, Eric Valette wrote:
> Hi,
> 
> I just bought an new external disk enclosure with e-sata/USB2 connector to replace an old USB2 only external disk enclosure.
> 
> My internal drive is a small SSD because I use the small factor PC as a home theater and do want silence and fast boot. I build my own kernel to put only what I need in and also boot somehow faster.
> 
> I use grub2 (up to date) as a loader. As soon as I power on the external sata disk, the system does not boot because hardware designer add the great idea to put the internal sata connector on ata3 whereas the external e-sata is wired on ata2.
> 
> consequence is that root file system is on sda1 when no external drive is connected and sdb1 when the external drive is connected.
> 
> As my grub setup was having a root=/dev/sda1 command line parameter and it was advertised everywhere that root=UUID=xxxxx was now supported, I changed the line to discover that this only work if an initramfs was loaded because support is done entirely in used space! Reading init/do_mounts.c made it very clear.
> 
> I find it extremely curious to have to add an initramfs to support dynamic drive identification, especially when the BYID value is displayed during boot message.
> 
> The boot loader cannot do a better job than the kernel and its life may be even worse as bios value for the same disk may change according to boot disk priorities.
> 
> Any hint comment? Any other way to avoid using a ramdisk? Please CC me as I'm not subscribed.

Your only alternative is to set up your root partition as an mdraid volume (single-disk raid0 perhaps), mark it type 'fd' so the kernel can auto-assemble it, and specify "root=/dev/md1" (or whatever minor number you set it up as).

Of course, adding an mdraid shim in between your bare device and the filesystem can reduce performance and/or data integrity for no other gain, and slow booting by the time taken examining all partitions.

I actually set up the above once when the ide => libata change-over was pending, since I was determined to NOT use an initramfs.  It worked, but I don't recommend it.

I recently converted all of my other auto-assembly mdraid systems to initramfs w/ the new dracut tool.  Simple, flawless (for me), and fast.

HTH,

Phil
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