Re: [bug #39591] grub-mkconfig fails on btrfs raid

[Andrei Borzenkov]

В Fri, 31 Jul 2015 23:41:44 -0700
Konstantin Svist <address@hidden> пишет:

> On 07/31/2015 11:12 PM, Andrei Borzenkov wrote:
> > [Adding grub-devel for wider discussion]
> >
> > В Fri, 31 Jul 2015 07:13:03 +0000
> > Konstantin Svist <address@hidden> пишет:
> >
> >> Follow-up Comment #3, bug #39591 (project grub):
> >>
> >> GRUB_DISABLE_LINUX_UUID=true
> >>
> >> For some reason I preferred not using UUIDs when setting it up, can't 
> >> remember
> >> why exactly..
> >>
> > Is it really possible to mount btrfs if only a single device is given? As I 
> > understand how multi-device btrfs works, to mount it you need to inform 
> > kernel about all (or at least enough) components. Today this is normally 
> > done by scanning each device during boot, feeding it to kernel and asking 
> > kernel "are there enough devices to attempt mount". This works only with 
> > UUID due to the way it is implemented in user space (udev+systemd+dracut).
> >
> > Passing single individual device as root= will defeat this logic - as soon 
> > as device is present user space will believe everything is OK and will try 
> > to mount it and likely fails.
> >
> > IOW - multi-device btrfs must use UUID, at least until something is changed 
> > in Linux boot device detection logic.
> >
> > So the only question for me is - should we fail grub-mkconfig in your case 
> > or ignore GRUB_DISABLE_LINUX_UUID and continue? I tend to fail it with 
> > clear message.
> >
> 
> 
> Excerpt from
> https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices:
> 
> Once you create a multi-device filesystem, you can use any device in the
> FS for the mount command:
> 
> mkfs.btrfs /dev/sdb /dev/sdc /dev/sde

At which point kernel of course knows all devices that are part of new
filesystem.

> mount /dev/sde /mnt
> 

address@hidden:~/src/systemd> sudo ln -sf /dev/null 
/run/udev/rules.d/64-btrfs.rules
address@hidden:~/src/systemd> sudo rmmod btrfs
rmmod: ERROR: Module btrfs is not currently loaded
address@hidden:~/src/systemd> sudo losetup --show -f /tmp/loop0
/dev/loop0
address@hidden:~/src/systemd> sudo losetup --show -f /tmp/loop1
/dev/loop1
address@hidden:~/src/systemd> sudo blkid
...
/dev/loop0: UUID="9e0de080-b2a0-47a8-b9a0-5a3e12fedee2" 
UUID_SUB="d6f072bb-ce32-4d7b-8cc3-31ef4d15a3d8" TYPE="btrfs" 
/dev/loop1: UUID="9e0de080-b2a0-47a8-b9a0-5a3e12fedee2" 
UUID_SUB="e2fb510e-56ed-4a2f-bce4-e53c1387c8b8" TYPE="btrfs" 
address@hidden:~/src/systemd> sudo mount -t btrfs /dev/loop0 /mnt
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
address@hidden:~/src/systemd> sudo mount -t btrfs /dev/loop1 /mnt
address@hidden:~/src/systemd> 

> IOW, this is normal operation for Btrfs. Maybe not for other RAIDs...
> 

Mounting multi-device btrfs requires explicit cooperation from user
space. There is no magic kernel can use to know all other devices.