Re: [PATCH] use UUID to map system devices to grub devices

[Felix Zielcke]

Am Montag, den 10.08.2009, 14:46 +0200 schrieb Felix Zielcke:
> Am Montag, den 10.08.2009, 13:31 +0200 schrieb Robert Millan:
> > On Sat, Aug 08, 2009 at 07:13:20AM +0200, Felix Zielcke wrote:
> > > Am Freitag, den 07.08.2009, 21:22 +0200 schrieb Robert Millan:
> > > > On Fri, Aug 07, 2009 at 04:29:00PM +0200, Felix Zielcke wrote:
> > > > > Am Freitag, den 07.08.2009, 13:27 +0200 schrieb Robert Millan:
> > > > > > On Wed, Aug 05, 2009 at 08:18:29AM +0200, Felix Zielcke wrote:
> > > > > > > Am Dienstag, den 04.08.2009, 23:19 +0200 schrieb Robert Millan:
> > > > > > > > On Fri, Jul 31, 2009 at 06:26:51PM +0200, Felix Zielcke wrote:
> > > > > > > > > 
> > > > > > > > > If we'd do an arbitrary mapping then `grub-probe -t drive' 
> > > > > > > > > would show
> > > > > > > > > the wrong grub device.
> > > > > > > > > But except from this I think that would be okay.
> > > > > > > > 
> > > > > > > > We can never garantee that `grub-probe -t drive' will show the 
> > > > > > > > "right" drive,
> > > > > > > > at least on i386-pc, because we don't know how is BIOS going to 
> > > > > > > > order them.
> > > > > > > 
> > > > > > > Yes drive not, but the partition.
> > > > > > 
> > > > > > It's true, but we don't really make the distinction.  UUID search 
> > > > > > will find
> > > > > > a filesystem, which is in a partition (usually), and doesn't rely 
> > > > > > on partitions
> > > > > > being reliable.
> > > > > > 
> > > > > > That's fortunate!  It means we don't have to commit to partition 
> > > > > > numbers being
> > > > > > reliable, even if they are right now.
> > > > > > 
> > > > > > Because of this (unless I missed something), at the end of the day 
> > > > > > the
> > > > > > unreliability issue you described doesn't translate into any real 
> > > > > > problem
> > > > > > for us.  It just adds more to a problem we already solved.
> > > > > > 
> > > > > > > Unfortunately we don't have UUID support on every filesystem we 
> > > > > > > support
> > > > > > > like JFS. But I think it's not that commonly used.
> > > > > > 
> > > > > > Adding UUID support to new filesystems is very easy.  I did the 
> > > > > > first ones
> > > > > > with just 5-10 minutes of research and a few lines of coding.
> > > > > > 
> > > > > > Would you like to do JFS ?
> > > > > 
> > > > > I did it now for JFS.
> > > > > 
> > > > > I tried it now out with dos_part set to p + 2 with my find_by_uuid 
> > > > > patch
> > > > > and now I get a `no such partition' error on my dmraid device.
> > > > > So we can't use an arbitary mapping in grub_util_biosdisk_get_grub_dev
> > > > 
> > > > Why not?
> > > 
> > > well then we would need to change the partition table parsing code to
> > > use the same arbitary mapping for GRUB_UTIL && LINUX then
> > > grub_util_biosdisk_get_grub_dev and I think that's not really a proper
> > > solution.
> > 
> > Why?  In that specific situation, it seems:
> > 
> >   - We're unable to obtain partition numbers reliably
> > 
> >   - It doesn't matter, because the upper layer will sort that out
> 
> Well with the current design of util/hostdisk.c it doestn't work.
> I don't know how we can make an arbitary mapping work.
> 
> > Though, it'd be much better if we could obtain this information from Linux.
> > Did you figure out if the behaviour of that ioctl is a bug or is 
> > intentional?
> 
> Probable intentional, because device-mapper is also used for LVM for
> which HDIO_GETGEO really doestn't make sense.
> Someone sent a patch to implement it for device-mapper in 2006 to LKML
> [0] but it wasn't accepted.
> Now I noticed Andrew Morton said `block_device_operations now has a
> standalone `getgeo' method.', which doestn't say me anything if this is
> something we can use or if this is purely internal for the kernel.
> 
> [0] http://lkml.indiana.edu/hypermail/linux/kernel/0602.1/index.html#0729
> 

Oh and it's limited to 2 TiB on 32bit systems, so I think it would be
good if we find an alternative to it.
Western Digital and Seagate sell now 2 TB disks.

-- 
Felix Zielcke
Proud Debian Maintainer