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Re: workaround install boot on btrfs with windows partition scheme

From: Andrei Borzenkov
Subject: Re: workaround install boot on btrfs with windows partition scheme
Date: Mon, 3 Nov 2014 08:36:37 +0300

В Sun, 2 Nov 2014 15:19:43 -0700
Chris Murphy <address@hidden> пишет:

> On Nov 1, 2014, at 11:37 PM, Andrei Borzenkov <address@hidden> wrote:
> > В Sat, 1 Nov 2014 19:34:56 -0600
> > Chris Murphy <address@hidden> пишет:
> > 
> >> 
> >> On Oct 30, 2014, at 6:42 AM, Andrei Borzenkov <address@hidden> wrote:
> >>> 
> >>> I still believe this is more flexible; in particular, /boot/grub on
> >>> btrfs has problems with unwritable grubenv (quite a few people are hit
> >>> by this now, when openSUSE defaults to single btrfs partition) so
> >>> having separate /boot as ext2 makes sense.
> >> 
> >> Hmm, interesting. What's the nature of this problem with grubenv on btrfs? 
> >> Is the current grubenv code expecting the file to be contiguous, and due 
> >> to COW on btrfs it ends up not being contiguous? Does setting xattr +C on 
> >> grubenv fix the problem?
> >> 
> > 
> > btrfs code does not execute hooks so block list collection always gives
> > empty block list and file size 0. That's unfortunate, because it also
> > means e.g. progress display is not possible.
> > 
> > btrfs data is checksummed so overwriting it would involve recomputing
> > checksums and replacing them. It is not possible to do in place because
> > checksums are checksummed themselves ... you get the idea.
> > 
> > btrfs can be on multiple devices so we cannot even expect grubenv to be
> > located on single disk; and blocklists simply do not support it.
> > 
> > btrfs can be compressed so you cannot even expect that new data will
> > fit into existing space.
> All the more reason for it to go in its own partition, rather than in some 
> tiny area, or multiple tiny areas in filesystems. It's taken years to 
> coordinate these things.
> > 
> >> Having separate /boot on ext was fine as a short/medium term work around, 
> >> but /boot on btrfs has use case benefits so it really needs to work 
> >> eventually.
> >> 
> > 
> > So far nobody suggested solution for grubenv on unwritable location.
> > Not to mention implementation …
> Put grubenv on 0xEA/BIOSboot as well. 

Please provide scheme how grub can reliably identify which of multiple
grub partitions on multiple disks is *the* partition where grubenv is

And it still should work when embedding bootloader in partition. Both
with and without blocklists.

>It's GRUB's partition it can put anything it wants there, exactly where
it expects to always find it. No coordination with filesystem groups
required. I don't see any of the fundamental behaviors about Btrfs
you're referring to changing anytime soon because it's simply how the
fs works, they aren't bugs. So either GRUB gets its own partition with
exclusive domain, or it continues to be hobbled by filesystem
dependencies like needing to be forced installed on ext, can't be
installed at all to XFS, and barely fits in 64KB on the Btrfs pad.
> It's not like there's a shortage of partitions.

There is in real life.

> And for future proofing this, enable GRUB to use more than one such
 partition. So if there's a 1MB 0xEA/BIOSboot and one day more space is
 needed, just add another one.
> Chris Murphy
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