[Nix-dev] NILFS2, NixOS, nixpart and Blivet

[Mateusz Kowalczyk]

On 09/30/2014 12:08 PM, Tim Barbour wrote:
> NILFS2 is a log-structured filesystem which is now in the Linux kernel source
> tree, and supported by GRUB2. It should appeal to NixOS users because it
> avoids destructive update ("changing" a file produces a new version of the
> file).
> 
> I have installed NixOS with all filesystems on NILFS2, but encountered some
> problems along the way.
> 
> The most serious problem is that NILFS2 needs to update /etc/mtab when
> mounting a filesystem, so that it can store information about the
> nilfs_cleanerd process associated with the mounted filesystem. If it cannot
> write to /etc/mtab, it does not start nilfs_cleanerd, and thus does not do
> garbage collection. In NixOS, /etc/mtab is just a symlink to /proc/mounts. I
> have worked around this manually so far, by replacing the symlink with a copy
> of /proc/mounts, then remounting all the NILFS2 filesystems. I am not sure how
> to resolve this properly. It is possible to mount using the '-n' option, but
> then NILFS2 will not start nilfs_cleanerd; it can be started later, but that
> requires writing to /etc/mtab. Perhaps the right way would be to mount using
> '-n', then start nilfs_cleanerd separately, but care would be necessary to
> make sure that nilfs_cleanerd is started exactly once (per filesystem), and
> shutdown appropriately at unmount.
> 
> Another problem is that nixpart does not work with NILFS2, because Blivet does
> not support it. I modified Blivet (0.41) to support NILFS2, but the NixOS
> Blivet packages use a very old version (0.17-1), so I cannot easily test my
> change against the latest version of Blivet. I have back-ported it to 0.17-1,
> and used that to setup partitions and filesystems for the above NixOS
> installation. I noticed one problem: it did not label any of the volumes, not
> even swap, even though I specified labels. I suspect this is a deficiency of
> Blivet 0.17-1, and imagine it is already fixed in Blivet 0.41 .
> 
> I considered updating the NixOS Blivet package(s), but they do some
> tricky-looking processing on the source code, and I doubt that I could update
> that properly for the current source code. If the maintainer could update the
> Blivet packages, then I would test my patch against the current Blivet, and
> send it to the upstream maintainers.
> 
> I attach a tarball with patches for Blivet (note that the 0.41 patch is
> untested).
> 
> Tim
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> 
> 
> 

Looks to me that all the scary-looking block in the blivet package does
is replace hardcoded paths with paths to the nix-store that nix provides
it with, a common practice. I think if you build and try to run the
newer version in chroot, it should quickly become apparent what new
changes have to be made for 0.41.

If someone doesn't step up then I think you might actually be the most
qualified person to do the update. Notably there's no listed maintainer
for the blivet package.

-- 
Mateusz K.