Re: Several questions regarding btrfs

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-11-01 13:52, Andrei Borzenkov wrote:
> 01.11.2017 15:01, Austin S. Hemmelgarn пишет:
> ...
>> The default subvolume is what gets mounted if you don't specify a
>> subvolume to mount.  On a newly created filesystem, it's subvolume ID 5,
>> which is the top-level of the filesystem itself.  Debian does not
>> specify a subvo9lume in /etc/fstab during the installation, so setting
>> the default subvolume will control what gets mounted.  If you were to
>> add a 'subvolume=' or 'subvolid=' mount option to /etc/fstab for that
>> filesystem, that would override the default subvolume.
>>
>> The reason I say to set the default subvolume instead of editing
>> /etc/fstab is a pretty simple one though.  If you edit /etc/fstab and
>> don't set the default subvolume, you will need to mess around with the
>> bootloader configuration (and possibly rebuild the initramfs) to make
>> the system bootable again, whereas by setting the default subvolume, the
>> system will just boot as-is without needing any other configuration
>> changes.
> 
> That breaks as soon as you have nested subvolumes that are not
> explicitly mounted because they are lost in new snapshot.
> 
Unless they have been created manually, there won't be any such 
subvolumes on a Debian system.  Debian treats BTRFS no different from 
any other filesystem during the install, so you get no subvolumes 
whatsoever (in contrast to Fedora and SUSE treating BTRFS as a volume 
manager and not a filesystem, and thus having subvolumes all over the 
place in a default install).

Regardless of if you update /etc/fstab to point to the new subvolume or 
not, any old ones need to be either copied (the preferred method for 
stuff that isn't supposed to be equivalent to a separate filesystem), or 
have entries put in /etc/fstab.