While digging deeper into the "subvolume list" command, I found
some flaws which I'm addressing here:
1. The "parent id" seems to be a relict of old days.
btrfs-subvolume(8) states:
"If -p is given, then parent <ID> is added to the output between
ID and top level. The parent's ID may be used at mount time via
the subvolrootid= option."
The "Mount Options" site [1] tells me that the "subvolrootid="
mount option is deprecated since kernel 3.2. A quick check on my
system (kernel 4.2.2) shows that "parent" is equal to "top level"
for all subvolumes. Is this always true for kernels >3.2 ?
Furthermore, the output is confusing, as "parent" is not related
at all to "parent_uuid".
Note that this is also true for "subvolume show", which
prints "parent" as "Parent ID".
Suggestion: drop the '-p' option and don't print "parent"
anymore. I don't know the "backwards compatibility policy" the
btrfs-devs have here, but my impression is that kernel 3.2 is not
really supported anymore.
2. The output of -a option (as well as -o) is inconsistent. Given
/tmp/btr_test is mounted with subvolid=0:
# btrfs subvolume create /tmp/btr_test/vol0
# btrfs subvolume create /tmp/btr_test/vol0/vol1
# btrfs subvolume create /tmp/btr_test/vol0/vol1/vol2
# list_path=/tmp/btr_test
# btrfs subvolume list -a $list_path
ID 256 gen 8 top level 5 path vol0
ID 257 gen 9 top level 256 path <FS_TREE>/vol0/vol1
ID 258 gen 9 top level 257 path <FS_TREE>/vol0/vol1/vol2
The only relative path shown is "vol0", which is
at "$list_path/vol0", as expected. But then "vol0/vol1" are
printed as absolute, while they are relative to $list_path.
# list_path=/tmp/btr_test/vol0
# btrfs subvolume list -a $list_path
ID 256 gen 8 top level 5 path <FS_TREE>/vol0
ID 257 gen 9 top level 256 path vol0/vol1
ID 258 gen 9 top level 257 path <FS_TREE>/vol0/vol1/vol2
But if $list_path points to a subvolume, the relative path shown
is now "vol0/vol1", which is actually at "$list_path/../vol0/vol1".
Things get even more confusing when subvolumes are within
directories:
# mkdir /tmp/btr_test/vol0/dir1
# btrfs subvolume create /tmp/btr_test/vol0/dir1/vol3
# list_path=/tmp/btr_test/vol0/dir1
# btrfs subvolume list -a $list_path
ID 256 gen 11 top level 5 path <FS_TREE>/vol0
ID 257 gen 9 top level 256 path vol0/vol1
ID 258 gen 9 top level 257 path <FS_TREE>/vol0/vol1/vol2
ID 259 gen 11 top level 256 path vol0/dir1/vol3
I know that handling "real" directories is very tricky to handle
correctly (involving symlinks et all), and is probably not a
feature that "subvolume list" command should handle.
Suggestions:
- Fix '-a' option to also print sub-subvolumes as relative, or
drop it (there is the -o option for this purpose).
- Fix output of '-o' option, by always printing relative
paths ("vol1" instead of "vol0/vol1" in the example above).
Regards,
- Axel
[1] https://btrfs.wiki.kernel.org/index.php/Mount_options
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