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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: Sun, 2 Nov 2014 08:37:01 +0300

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.

> 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 ...

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