[arch-general] btrfs & Arch

[Simon Schneider]

2011/8/25 C Anthony Risinger <anthony at xtfx.me>

> On Thu, Aug 25, 2011 at 2:19 AM, Gour-Gadadhara Dasa <gour at atmarama.net>
> wrote:
> > On Wed, 24 Aug 2011 12:44:10 -0500
> > C Anthony Risinger <anthony at xtfx.me> wrote:
> >
> >> unfortunately you can only do /boot on btrfs if you use a single disk
> >> due to bootloader limitations:
> >
> > Is it only bootloader limitation considering that wiki page says: "This
> setup
> > does not support btrfs RAID because a separate boot partition/device is
> needed,
> > ie. only one drive allowed. Pending releases of mkinitcpio-btrfs will
> attempt
> > to provide solutions." ?
>
> im not sure im understanding correctly ... mkinitcpio-btrfs is just an
> initcpio hook performing a handful of trixy things to "rollback in
> time", ie. boot an older snapshot.  everything it does is orthogonal
> to the /boot problem ... though i had some experimentations using a
> two-stage system that booted a minimal kernel far enough to load the
> real kernel off the btrfs array -- along with other activity this
> could "enable" /boot on a btrfs array -- not even tested yet though.
>
> fundamentally syslinux and friends (AFAIK) are not capable of reading
> a disk if part of an array.  syslinux can't even look inside a
> subvolume ... hence kernel rollbacks are not straightforward.
>
> you *can* however have / on btrfs, *and* an array ... so long as your
> /boot is still using `md` + <other FS>.  maybe btrfs could even be
> that FS ... not sure.  for this to work you just need the `btrfs` hook
> included with Arch, or you need to pass all the devices in the array
> as a mount option (eg. via `rootflags` on kernel bootline)
>
> --
>
> C Anthony
>

I got even more curious about btrfs after following this discussion and
installed it on my primary laptop (I planned a reinstall anyway to get rid
of windows). Installation worked perfectly with Archboot, setting up two
subvolumes, both compressed and everything worked perfectly for about a
week: My DVR stick made the computer freeze (this happens like twice a year)
and therfore I had to hard reset the machine with the effect that it
rendered the filesystem unusable for the linux kernel. Grub could still read
the kernel image and booted it just fine, but as soon as the kernel tried to
mount the filesystem it crashed. I tried to recover with a live cd, but
because I had no internet access at that time, and man pages weren't very
helpful either I eventuelly just reinstalled Arch using the familiar lvm2 +
ext4 setup.

Btrfs is great, don't get me wrong, it really is fast and snapshots and all
that worked really great, but it is, sadly, a little bit too unstable to use
it anywhere than on a test machine. Even if you have UPS and all kind of
measurements to prevent such power failures and hard resets, your computer
still might hang at some point and then you'll really are in big troubles.