btrfs and kickstart re-installs

[Gene Czarcinski]

In general, kickstart has good support for installing onto btrfs 
subvolumes.  However, there is one case that has a small problem: 
re-installing onto an existing subvol rootfs and/or /boot.

To do an install,  anaconda requires that the rootfs and /boot be 
"freshly formatted."  For regular partitions and LVMlv, this presents no 
problem, just reformat the partition or LVMlv.  On the other hand, a 
btrfs subvolume more closely resembles a directory than a partition and 
how do you re-format a directory? ... well, by deleting it and then 
re-defining it.  I can easily do this when I do a regular gui install 
but kickstart is another matter.

I proposed an update to anaconda/pykickstaert to add the "--reclaim" 
option for the btrfs command.  This was rejected by anaconda developers 
because it did not fit with anaconda architecture (my words).  An 
alternative was proposed to use clearpart to delete the subvolume by 
referring to the subvolume by it unique UUID.  I do not find this useful 
because I cannot rerun a kickstart install without changing the 
kickstart file (each run will involve a different UUID).

I do not want to carry my local modification so I came up with an 
alternative that requires no modification, works, and works if re-run 
without changing the kickstart file.

Solution:  I use a %pre section to delete the subvolume.  Given a system 
with /boot on a ext4 partition and an existing btrfs volume supporting 
root6 and home subvolumes, it would look something like:

###############
clearpart --none
...
part swap  --fstype=swap --noformat 
--onpart=UUID=a3b0c2ad-88c8-4d6d-876c-7c52881ab854

#part /boot --fstype=ext4 --onpart=sda4
#part /boot --fstype=ext4 --onpart=sda5
part /boot --fstype=ext4 --onpart=sda6
#part /boot --fstype=ext4 --onpart=sda7

part btrfs.10 --fstype=btrfs --noformat --label=ten 
--onpart=UUID=0f0f82cb-ed27-4468-a18c-64a877b64329
btrfs /                       --subvol --name=root6   ten
btrfs /var                    --subvol --name=var6    ten
btrfs /home                   --subvol --name=home --noformat  ten
...
%pre --log=/tmp/pre-install.log
echo "manage the btrfs subvolumes"
mkdir /mnt/btrfsvol
mount UUID=0f0f82cb-ed27-4468-a18c-64a877b64329 /mnt/btrfsvol
btrfs subvol list /mnt/btrfsvol
btrfs subvol delete /mnt/btrfsvol/root6
btrfs subvol delete /mnt/btrfsvol/var6
btrfs subvol list /mnt/btrfsvol
umount /mnt/btrfsvol
rmdir /mnt/btrfsvol
%end

%post --nochroot --log=/mnt/sysimage/root/post-install.log
cp -v /tmp/pre-install.log /mnt/sysimage/root
%end
#############

The first time this is run, the "btrfs subvol delete" will produce 
errors.  Thereafter, reruns will delete the subvols so they can be 
re-installed.

Note:  Besides this mailing list, this might be good information to add 
to the kickstart documentation.

Gene
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