Re: btrfs subvolume naming scheme
On Sat, Apr 23, 2016 at 05:51:25PM -0400, Nicholas D Steeves wrote:
> On 23 April 2016 at 07:38, New Thread old subject joining team
> <stappers@stappers.nl> wrote:
> > On Sat, Apr 23, 2016 at 01:28:43PM +0200, Philipp Kern wrote:
> >> On Fri, Apr 22, 2016 at 08:30:35PM -0400, Nicholas D Steeves wrote:
> >>
> >> > I'd also like to discuss whether the default subvolume naming scheme
> >> > should follow Ubuntu, Fedora, OpenSUSE, or something else.
> >>
> >> What scheme are they using?
> >
> > Or a proposal for default subvolume naming scheme?
> >
>
> Ubuntu avoids using the default subvolume (subvol ID 5). For the
> rootfs their installer creates a subvol called @, for /home it creates
> @home, etc. In fstab the device is specified and subvol=@ is added to
> the mount option to specify which subvolume gets mounted. When the
> volume is mounted without a subvol option, it mounts the whole tree.
> The tree would be /btrfs/@ and /btrfs/@home if mounted from a rescue
> disk.
>
> I think the symbol is visually striking, but it can be cumbersome
> because @+<tab> sometimes autocompletes as ipv6 addresses for root.
> I'm also not sure if @ ever needs to be escaped \@.
FWIW I have no memory of @ needed to be escape.
A websearch on "linux shell when needs @ be escaped \@"
( https://www.google.nl/search?q=linux+shell++when+does+%40+needed+to+be+escaped+\%40 )
didn't show me that @ is special for bash.
Doing @+<tab> for autocompletion is done interactive by a thinking user,
so I see no danger, I trust the thinking user.
> Oh! I just booted OpenSUSE Leap (their LTS), and it looks like
> they've now adopted the Ubuntu convention of using @. OpenSUSE has
> also, to my knowledge, always avoided using the default subvolume.
> Furthermore, OpenSUSE creates subvolumes for just about everything
> @opt, @srv, @tmp, @usr/local, @var/crash, etc.
>
> Fedora 23 Workstation: When btrfs-style partitioning is selected,
> their installer creates two subvolumes, home, and root. When the
> volume is mounted to /btrfs without a subvol= option, the tree would
> be /btrfs/home and /btrfs/root. Like Ubuntu and OpenSUSE default
> subvolume is also not used. From what I gather this is a necessary
> configuration to support btrfs send and receive. Eg: Bug #764056 is a
> result of our current policy.
>
> Unlike LVM or disk partitions, all free space is shared between
> subvolumes. In the future it will be possible to use qgroups (quota
> groups) to prevent /var or /home from using up all available free
> space in rootfs, but at this time I don't think we should support it
> in the installer, because of the volume of associated bugs and code
> churn on the linux-btrfs mailing list. Also, in the future it will be
> possible to mount subvolumes with different options, but at this time
> the first subvolume mounted sets the mount options for all members of
> the volume--I'm not sure how to address this the D-I.
>
> In consultation with
> https://btrfs.wiki.kernel.org/index.php/SysadminGuide#When_To_Make_Subvolumes
> , a subvolume for rootfs and for /home seems sane, and sysadmins can
> be instructed to consider making a subvolume for /var/www in
> documentation.
>
> This brings us to a concern I have for documentation. How should
> /var/www appear when it's mounted using a rescue disk? Should it be
> /btrfs/var_www? If it was /btrfs/var/www, then the two possibilities
> are:
>
> a) /btrfs/var is its own subvolume
> and /btrfs/var/www is a child subvolume of /btrfs/var
>
> note: strictly speaking, all subvolumes what seems to be the root
> volume are actually children of the default subvolume...the semantics
> get tricky very quickly!
>
> or
>
> b) /btrfs/var is a normal directory
> and in the case of /btrfs/var/www, www is actually a child of /btrfs.
>
> Finally, because subvolumes are partitions in POSIX namespace, it's
> safe to mount a subvolume to two locations, and also to have a
> /btrfs-admin directory where the whole volume is mounted, at the same
> time as individual subvolumes are mounted. eg: you have your rootfs
> mounted at /, and also at /btrfs-admin/rootfs or /btrfs-admin/@.
>
> The primary reason to do this is because most of the btrfs tools
> operate on mountpoints rather than on devices. It also allows
> centralisation of snapshots. eg: /btrfs-admin/snapshots is a normal
> directory that holds snapshots of /btrfs-admin/rootfs,
> /btrfs-admin/home, etc.
>
> Résumé: Do we follow Ubuntu and OpenSUSE with the @ convention and
> work through the issues in bash-completion, or we follow Fedora's
> plain text/alphanumeric convention, or do we do our own thing?
The one which follows BTRFS upstream philosophy.
However, I don't know if such guide exists.
> Secondly, Do we want to limit the difficulty of supporting complicated
> configurations by establishing simple conventions and recommendation
> early on? eg: all subvolumes created in the installation are peers,
> and a subvolume that will be mounted at /var/www is named var_www.
> A default delimiter convention would also need to be chosen.
Install a starting point.
More complicated configuration can be done on the installed system.
KISS
> I look forward to your replies,
> Cheers!
> Nicholas
Groeten
Geert Stappers
--
Leven en laten leven
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