Hi Florian,
in case you would like to have some code references, to better
understand how it works:
1. Why is the default subvolid=5?
The trees in Btrfs all are referenced in the root tree by their
OBJECTIDs. You can find the definitions("magic numbers") for the
"main" trees in ctree.h:
For example, the main FS tree (you have one FS tree per subvolume) is
defined like this:
#define BTRFS_FS_TREE_OBJECTID 5ULL
==> subvolid=5
2. Why can I mount with -o subvolid=0 succesfully, albeit the default
FS tree has subvolid=5?
Have a look at: linux/fs/btrfs/super.c
In early mount options parser: btrfs_parse_early_options
Snippet:
case Opt_subvolid:
intarg = 0;
error = match_int(&args[0], &intarg);
if (!error) {
/* we want the original fs_tree */
/*Here is the trick ==> */ if (!intarg)
*subvol_objectid =
BTRFS_FS_TREE_OBJECTID;
else
*subvol_objectid = intarg;
}
break;
==> So, if intarg is still 0 after the match_int line, it will use
BTRFS_FS_TREE_OBJECTID (which we know from 1. being 5ULL) when
mounting.
Best Regards,
Alex
2012/7/30 Florian Lindner <mailinglists@xxxxxx>:
> Hey!
>
> I recently starting playing with btrfs and subvolume, but it has left
> me puzzled:
> Distribution is Archlinux, Kernel is 3.4.6.
>
>>>>
> root@horus /mnt # mkfs.btrfs -L test /dev/sdb1
>
> WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
> WARNING! - see http://btrfs.wiki.kernel.org before using
>
> fs created label test on /dev/sdb1
> nodesize 4096 leafsize 4096 sectorsize 4096 size 2.73TB
> Btrfs Btrfs v0.19
> root@horus /mnt # mount /dev/sdb1 test
> root@horus /mnt # cd test
> root@horus /mnt/test # btrfs subvolume create sv1
> Create subvolume './sv1'
> root@horus /mnt/test # btrfs subvolume create sv2
> Create subvolume './sv2'
> root@horus /mnt/test # touch sv1/sv1.file
> root@horus /mnt/test # touch sv2/sv2.file
> root@horus /mnt/test # btrfs subvolume get-default .
> ID 256 top level 5 path sv1
> ID 259 top level 5 path sv2
> <<<
>
> What is the default subvolume now? How can I tell?
>
>>>>
> root@horus /mnt/test # btrfs subvolume set-default 259 .
> root@horus /mnt/test # btrfs subvolume get-default .
> ID 256 top level 5 path sv1
> ID 259 top level 5 path sv2
> <<<
>
> Seems to have changed nothing....?
>
>>>>
> root@horus /mnt/test # cd ..
> root@horus /mnt # umount test && mount /dev/sdb1 test
> root@horus /mnt # ls test
> sv2.file
> <<<
>
> Ah, sv2 seems to be default, like I had set it.
>
>>>>
> root@horus /mnt # btrfs subvolume set-default 5 test
> root@horus /mnt # umount test && /mnt # mount /dev/sdb1 test
> root@horus /mnt # ls test
> sv1/ sv2/
> <<<
>
> Ok, 5 seems to be the root subvolume id. Is it always like that? I
> remembered to have read somewhere it was 0 ? (which makes a kind of
> more sense for me)
>
>>>>
> root@horus /mnt # btrfs subvolume set-default 256 test
> root@horus /mnt # umount test && mount /dev/sdb1 test
> root@horus /mnt # ls test
> sv1.file
> <<<
>
> Fine! But:
>
>>>>
> root@horus /mnt # btrfs subvolume set-default 0 test
> root@horus /mnt # umount test && mount /dev/sdb1 test
> root@horus /mnt # ls test
> sv1.file
> <<<
>
> set-default 0 seems to do nothing but does not produce an error
> either. What about subvolume 0? Still I can do:
>
>>>>
> root@horus /mnt # umount test
> root@horus /mnt # mount -o subvolid=0 /dev/sdb1 test
> root@horus /mnt # ls test
> sv1/ sv2/
> <<<
>
> Ok, here 0 as subvolid works. What about subvolid=5?
>
>>>>
> root@horus /mnt # umount test
> root@horus /mnt # mount -o subvolid=5 /dev/sdb1 test
> root@horus /mnt # ls test
> sv1/ sv2/
> <<<
>
> Works too.
>
> Sorry for the lengthy posting, but writing this posting has puzzled me
> even more I was yesterday. I hope someone could shed some light on it.
>
> Thanks!
>
> Florian
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