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 > <stapp...@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
