Re: [PATCH] i386-pc: build btrfs zstd support into separate module

From: Michael Chang <mchang@suse.com>
To: The development of GNU GRUB <grub-devel@gnu.org>
Subject: Re: [PATCH] i386-pc: build btrfs zstd support into separate module
Date: Fri, 10 Sep 2021 17:22:22 +0800	[thread overview]
Message-ID: <20210910092222.GA11439@mazu> (raw)
In-Reply-To: <20210908193752.u4pkfbkommbjkazq@tomti.i.net-space.pl>

On Wed, Sep 08, 2021 at 09:37:52PM +0200, Daniel Kiper wrote:
> On Fri, Sep 03, 2021 at 09:21:39AM +0800, Michael Chang via Grub-devel wrote:
> > On Thu, Sep 02, 2021 at 02:12:52PM +0200, Daniel Kiper wrote:
> > > On Thu, Sep 02, 2021 at 01:48:30PM +0800, Michael Chang via Grub-devel wrote:
> > > > On Wed, Sep 01, 2021 at 06:38:22PM +0200, Daniel Kiper wrote:
> > > > > On Tue, Aug 31, 2021 at 03:12:28PM +0800, Michael Chang via Grub-devel wrote:

[snip]

> > just that I can't resist to fix problem from our users who opted to use
> > "btrfs partition" which differs to "short mbr gap" with a lot more user
> > base and sensible usecases.
> >
> > FWIW we want to address this problem, because mbr gap is adjustable via
> > re-partitioning but btrfs bootloader area is not.
> 
> Huh! The "btrfs partition" sounds like not resizable MBR gap! I am not
> happy with it just at the beginning. Anyway, could you explain your use
> case in more details including example commands and why core.img seems
> landing in the "btrfs partition". I am especially curious about the
> latter because I think the "btrfs partition" and things like that was
> designed for something else, e.g., storing grubenv data. And why this
> solution is i386 specific?

Installing to btrfs partition is not something exotic to grub, it is
actually a supported usecase by grub for a long time. Also zfs has
similar design, see:

  c7ba4f698     Support BtrFS embedding.
  ba102053c     Support ZFS embedding.

To make it happen, you just have to point the btrfs device to
grub-install and it will setup and embed image there. That worked quite
nicely until the zstd support came into play.

On current git head, the command fails with core.img is too large. 

# ./grub-install -d ./grub-core /dev/vda2
  Installing for i386-pc platform.
  ./grub-install: warning: your core.img is unusually large.  It won't fit in the embedding area.
  ./grub-install: error: filesystem `btrfs' doesn't support blocklists.

With this patch, the embedding still works but warns the user zstd is
disabled, also instructing them to use MBR if they need the zstd feature
to work.

# ./grub-install -d ./grub-core /dev/vda2
Installing for i386-pc platform.
./grub-install: warning: btrfs zstd compression is disabled, please change install device to disk.
Installation finished. No error reported.

It is very i386-pc centric, as it is often used by legacy mbr boot code
with which active partition is chainloaded. Some people still thinks
this is the right thing to do and feels more comfortable this way to
manage their multiple distrubution booting, compared with os-prober.

Thanks,
Michael

> 
> Daniel
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel