Re: "@" prefix in subvolume paths

[Jeff Mahoney <jeffm@suse.com>]

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On 6/10/15 7:10 PM, Hugo Mills wrote:
> On Tue, Jun 09, 2015 at 07:55:05PM +0200, pubny@xs4all.nl wrote:
>> Hi,
>> 
>> I've noticed this "@" sign in the subvolume's path with SLES12.
>> I'm wondering what is its purpose. openSUSE 13.2 doesn't seem to
>> be using it.
> 
> They created a subvolume called "@" and then put all the other 
> subvolumes under there. There's nothing special about it.
> 
> The @ is often used as a prefix on subvolumes (e.g. in Ubuntu) to 
> indicate that they're subvolumes, but it's not required, and
> doesn't have any extra meaning beyond being a useful naming
> convention.

Yep. That explanation is accurate but is missing the rest of the
story. The choice of @ is probably due to following the convention
started by the Ubuntu folks but also because it's the least
complicated of the alternative names we could have used. The @
subvolume is set as the default subvolume. As you'd expect, it's
mounted at / and used as such. The reason we use a separate subvolume
is so we can easily implement our boot-and-rollback-from-snapshot
functionality in SLE12. Having the root file system as a separate
subvolume means we can move one of the snapshot subvolumes into the fs
tree root, move the other @ subvolumes into that subvolume, and we
have a rolled back system from which we can easily remove the
now-unused root. If we didn't use a separate subvolume for it, it
would make the rollback very complicated. It's used in concert with
our GRUB implementation that iterates and presents possible snapshots
to use as a root file system in lieu of the "real" one should a
problem occur.

- -Jeff

>> 
>> SLES12: # btrfs sub list / ID 257 gen 2270 top level 5 path @ ID
>> 258 gen 1655 top level 257 path @/boot/grub2/i386-pc ID 259 gen
>> 4263 top level 257 path @/boot/grub2/x86_64-efi ID 260 gen 4037
>> top level 257 path @/opt ID 261 gen 2553 top level 257 path
>> @/srv ID 262 gen 4521 top level 257 path @/tmp ID 263 gen 4492
>> top level 257 path @/usr/local ID 264 gen 1655 top level 257 path
>> @/var/crash ID 265 gen 1655 top level 257 path @/var/lib/mailman 
>> ID 266 gen 1655 top level 257 path @/var/lib/named ID 267 gen
>> 1655 top level 257 path @/var/lib/pgsql ID 268 gen 4523 top level
>> 257 path @/var/log ID 269 gen 1655 top level 257 path @/var/opt 
>> ID 270 gen 4524 top level 257 path @/var/spool ID 271 gen 4521
>> top level 257 path @/var/tmp ID 275 gen 4505 top level 257 path
>> @/.snapshots
>> 
>> 
>> openSUSE 13.2:
>> 
>> # btrfs subvol list / ID 257 gen 3481911 top level 5 path
>> boot/grub2/i386-pc ID 258 gen 3481911 top level 5 path
>> boot/grub2/x86_64-efi ID 259 gen 3517714 top level 5 path home ID
>> 260 gen 3515789 top level 5 path opt ID 261 gen 3513493 top level
>> 5 path srv ID 262 gen 3513372 top level 5 path tmp ID 263 gen
>> 3517660 top level 5 path usr/local ID 264 gen 3513372 top level 5
>> path var/crash ID 265 gen 3513372 top level 5 path
>> var/lib/mailman ID 266 gen 3513372 top level 5 path
>> var/lib/named ID 267 gen 3513372 top level 5 path var/lib/pgsql 
>> ID 268 gen 3517714 top level 5 path var/log ID 269 gen 3513372
>> top level 5 path var/opt ID 270 gen 3517714 top level 5 path
>> var/spool ID 271 gen 3517706 top level 5 path var/tmp ID 276 gen
>> 3517669 top level 5 path .snapshots
> 


- -- 
Jeff Mahoney
SUSE Labs
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