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Bug#686130: still not booting from BTRFS RAID 1 on two LVs out of the box

On Freitag, 10. Juni 2016 00:04:04 CEST Ben Hutchings wrote:
> On Thu, 2016-06-09 at 18:24 +0200, Martin Steigerwald wrote:
> > With todays Debian Sid/Experimental on my laptop this still doesn´t work 
> > right, I still have to add something like – I just recognized after I 
> > reinstalled initramfs-tools in a crude way after having messed it up by 
> > accidentally delete /var/lib/initramfs-tools (and then recognizing that it
> > may 
> > have been easier to recover it than what I did):
> [...]
> > So whatever local-premount/btrfs does it not sufficient for the BTRFS RAID
> > on  two LVs case.
> Indeed.  As explained in #565676, lvm2 only activates the VGs
> containing the root, /usr and hibernate/resume devices (and only if it
> can recognise that they are on LVM).  Apparently it is not generally
> safe to activate all visible VGs.  But perhaps that behaviour should be
> available as an *option*.  That is safe for most systems and I think it
> would resolve all the issues blocking this.

Well, it would definately work on my system.

> (I don't really understand the purpose of running btrfs on LVM, though.
> btrfs integrates logical volume management and I was under the
> impression that that's the main reason to use it.  If you still need
> another layer underneath then why use it at all?)

The reason is simple: I want to have the option to use and test different 
filesystems for my Linux Performance Analysis and Tuning courses and other 
reasons. Also I have two BTRFS RAID 1, the debian system itself aka /, and 
home separately, and one data partition only residing on one SSD.

Once BTRFS allows different RAID levels  in each subvolume it would be 
possible to have this in a single BTRFS, but… I give up the separation of 
operating system and home device thus making it more difficult to reinstall 
the operating system in case it would be needed – I don´t know how good BTRFS 
support is in the installer meanwhile – and probably increasing risk of data 
loss in case of filesystem troubles. Right now when /-device would break, it 
doesn´t affect /home. Having both as a subvolume if one breaks it is more 
likely that it affects the other one as well.


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