root subvol id is 0 or 5?

root subvol id is 0 or 5?

[Shriramana Sharma]

I am confused with this: should I call it the "root subvol" or
"top-level subvol" or "default subvol" or doesn't it matter? Are all
subvols equal, or some are more equal than others [hark to Orwell's
Animal Farm ;-)]?

And more importantly, is the ID of the root subvol 0 or 5?

The Oracle guide
(https://docs.oracle.com/cd/E37670_01/E37355/html/ol_use_case3_btrfs.html)
seems to say it's 0 :

"By default, the operating system mounts the parent btrfs volume,
which has an ID of 0"

but the BtrFS wiki (and btrfs subvol manpage) reads 5:

"every btrfs filesystem has a default subvolume as its initially
top-level subvolume, whose subvolume id is 5(FS_TREE)."

as also the Ubuntu Wiki:

"The default subvolume to mount is always the top of the btrfs tree
(subvolid=5)."

Now this Oracle page
http://www.oracle.com/technetwork/articles/servers-storage-admin/advanced-btrfs-1734952.html
says:

"The only clean way to destroy the default subvolume is to rerun the
mkfs.btrfs command, which would destroy existing data."

So from what I've (confusedly) understood so far, 0 refers to the
superstructure (or whatchamacallit) of the entire BtrFS-based contents
of the device(s) and hence cannot be deleted but only reset by a
mkfs.btrfs, but 5 is only the default subvol (mounted when the FS as a
whole is mounted without subvol spec) provided by mkfs.btrfs, and
subvol set-default can have another subvol mounted as default instead,
after which 5 can actually be deleted?

[confused]...

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा

Re: root subvol id is 0 or 5?

[Goffredo Baroncelli]

On 11/30/2014 04:31 AM, Shriramana Sharma wrote:
> I am confused with this: should I call it the "root subvol" or
> "top-level subvol" or "default subvol" or doesn't it matter? Are all
> subvols equal, or some are more equal than others [hark to Orwell's
> Animal Farm ;-)]?
> 
> And more importantly, is the ID of the root subvol 0 or 5?

IIRC the real ID is 5. But if you pass 0, this value is converted in 5

$ grep BTRFS_FS_TREE_OBJECTID *.h
ctree.h:#define BTRFS_FS_TREE_OBJECTID 5ULL
[...]

And in super.c:

[...]
                case Opt_subvolid:
                        num = match_strdup(&args[0]);
                        if (num) {
                                *subvol_objectid = memparse(num, NULL);
                                kfree(num);
                                /* we want the original fs_tree */
                                if (!*subvol_objectid)
                                        *subvol_objectid =
                                                BTRFS_FS_TREE_OBJECTID;

[...]


-- 
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D  17B2 0EDA 9B37 8B82 E0B5

Re: root subvol id is 0 or 5?

[Hugo Mills]

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On Sun, Nov 30, 2014 at 09:01:37AM +0530, Shriramana Sharma wrote:
> I am confused with this: should I call it the "root subvol" or
> "top-level subvol" or "default subvol" or doesn't it matter? Are all
> subvols equal, or some are more equal than others [hark to Orwell's
> Animal Farm ;-)]?

   I try to use "top level" for subvolid=5.

   "root subvol" is hugely confusing, as it could be one of several
things. If you mean the subvol mounted at /, then I call that "/" or
"the / subvol".

   "default subvol" is the one marked as default. This starts out as
subvolid=5, but can be set to any other subvol.

> And more importantly, is the ID of the root subvol 0 or 5?

   In the data structures on disk, it's 5. The kernel aliases 0 to
mean subvolid 5.

> The Oracle guide
> (https://docs.oracle.com/cd/E37670_01/E37355/html/ol_use_case3_btrfs.html)
> seems to say it's 0 :
> 
> "By default, the operating system mounts the parent btrfs volume,
> which has an ID of 0"
> 
> but the BtrFS wiki (and btrfs subvol manpage) reads 5:
> 
> "every btrfs filesystem has a default subvolume as its initially
> top-level subvolume, whose subvolume id is 5(FS_TREE)."
> 
> as also the Ubuntu Wiki:
> 
> "The default subvolume to mount is always the top of the btrfs tree
> (subvolid=5)."

   As above, both are correct here.

> Now this Oracle page
> http://www.oracle.com/technetwork/articles/servers-storage-admin/advanced-btrfs-1734952.html
> says:
> 
> "The only clean way to destroy the default subvolume is to rerun the
> mkfs.btrfs command, which would destroy existing data."

   OK, this is actually wrong. It's not the default subvolume if
someone's run set-default on the FS. They're correct that you can't
delete the top-level subvol. You can't delete the subvol marked as
default, either. Assuming (or implying) that the two are the same is
just plain wrong.

> So from what I've (confusedly) understood so far, 0 refers to the
> superstructure (or whatchamacallit) of the entire BtrFS-based contents
> of the device(s) and hence cannot be deleted but only reset by a
> mkfs.btrfs, but 5 is only the default subvol (mounted when the FS as a
> whole is mounted without subvol spec) provided by mkfs.btrfs, and
> subvol set-default can have another subvol mounted as default instead,
> after which 5 can actually be deleted?

   You can't delete subvolid=5. It's part of the "fundamental
whatchamacallit" of the FS (a good name). Even if you change the
default subvol, you still can't delete it.

   Hugo.

-- 
Hugo Mills             | People are too unreliable to be replaced by
hugo@... carfax.org.uk | machines.
http://carfax.org.uk/  |
PGP: 65E74AC0          |                              Nathan Spring, Star Cops

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Re: root subvol id is 0 or 5?

[Shriramana Sharma]

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On Sun, Nov 30, 2014 at 5:29 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
>
>    In the data structures on disk, it's 5. The kernel aliases 0 to
> mean subvolid 5.

So why 5 and not just 0 which seems a logical choice? On top of this,
one needs to alias 0 to 5!

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा
ÿôèº{.nÇ+‰·Ÿ®‰­†+%ŠËÿ±éݶ\x17¥Šwÿº{.nÇ+‰·¥Š{±ý»k~ÏâžØ^n‡r¡ö¦zË\x1aëh™¨è­Ú&£ûàz¿äz¹Þ—ú+€Ê+zf£¢·hšˆ§~†­†Ûiÿÿïêÿ‘êçz_è®\x0fæj:+v‰¨þ)ߣøm

Re: root subvol id is 0 or 5?

[Shriramana Sharma]

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On Sun, Nov 30, 2014 at 7:08 PM, Shriramana Sharma <samjnaa@gmail.com> wrote:
>
> So why 5 and not just 0 which seems a logical choice? On top of this,
> one needs to alias 0 to 5!

Attached patch clarifying this in the documentation. (Should have done
this with the previous mail. Sorry for multiple mails.)

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा

[-- Attachment #2: 0002-btrfs-subvolume-doc-clarifications.patch --]
[-- Type: text/x-patch, Size: 1332 bytes --]

From 54387ff2155423d990b5a9aca95315fe6e649303 Mon Sep 17 00:00:00 2001
From: Shriramana Sharma <samjnaa@gmail.com>
Date: Sun, 30 Nov 2014 19:11:39 +0530
Subject: [PATCH 2/2] btrfs subvolume doc clarifications

---
 Documentation/btrfs-subvolume.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/btrfs-subvolume.txt b/Documentation/btrfs-subvolume.txt
index 1360aba..34abdef 100644
--- a/Documentation/btrfs-subvolume.txt
+++ b/Documentation/btrfs-subvolume.txt
@@ -31,7 +31,7 @@ When `mount`(8) using 'subvol' or 'subvolid' mount option, one can access
 files/directories/subvolumes inside it, but nothing in parent subvolumes.
 
 Also every btrfs filesystem has a default subvolume as its initially top-level
-subvolume, whose subvolume id is 5(FS_TREE).
+subvolume, whose subvolume id is 5. (0 is also acceptable as an alias.)
 
 A btrfs snapshot is much like a subvolume, but shares its data(and metadata)
 with other subvolume/snapshot. Due to the capabilities of COW, modifications
@@ -166,7 +166,7 @@ sleep N seconds between checks (default: 1)
 
 EXIT STATUS
 -----------
-*btrfs subvolume* returns a zero exit status if it succeeds. Non zero is
+*btrfs subvolume* returns a zero exit status if it succeeds. A non-zero value is
 returned in case of failure.
 
 AVAILABILITY
-- 
2.1.3

Re: root subvol id is 0 or 5?

[David Sterba]

On Sun, Nov 30, 2014 at 07:12:46PM +0530, Shriramana Sharma wrote:
> On Sun, Nov 30, 2014 at 7:08 PM, Shriramana Sharma <samjnaa@gmail.com> wrote:
> >
> > So why 5 and not just 0 which seems a logical choice? On top of this,
> > one needs to alias 0 to 5!
> 
> Attached patch clarifying this in the documentation. (Should have done
> this with the previous mail. Sorry for multiple mails.)
> 
> -- 
> Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा

> From 54387ff2155423d990b5a9aca95315fe6e649303 Mon Sep 17 00:00:00 2001
> From: Shriramana Sharma <samjnaa@gmail.com>
> Date: Sun, 30 Nov 2014 19:11:39 +0530
> Subject: [PATCH 2/2] btrfs subvolume doc clarifications

Applied, thanks.

Re: root subvol id is 0 or 5?

[Hugo Mills]

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On Sun, Nov 30, 2014 at 07:08:51PM +0530, Shriramana Sharma wrote:
> On Sun, Nov 30, 2014 at 5:29 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
> >
> >    In the data structures on disk, it's 5. The kernel aliases 0 to
> > mean subvolid 5.
> 
> So why 5 and not just 0 which seems a logical choice? On top of this,
> one needs to alias 0 to 5!

   All of the trees used in the FS metadata have an ID number. The
"well-known" trees have small, fixed IDs:

#define BTRFS_ROOT_TREE_OBJECTID 1ULL
#define BTRFS_EXTENT_TREE_OBJECTID 2ULL
#define BTRFS_CHUNK_TREE_OBJECTID 3ULL
#define BTRFS_DEV_TREE_OBJECTID 4ULL
#define BTRFS_FS_TREE_OBJECTID 5ULL
#define BTRFS_ROOT_TREE_DIR_OBJECTID 6ULL
#define BTRFS_CSUM_TREE_OBJECTID 7ULL
#define BTRFS_QUOTA_TREE_OBJECTID 8ULL
#define BTRFS_UUID_TREE_OBJECTID 9ULL

   Note that "the" FS tree has ID 5. A subvolume is basically another
FS tree. Subvolumes other than the top level are given dynamically-
allocated IDs starting from 256. Note also that the root, chunk,
device and extent trees are all more important, lower level
information than any FS tree, so they logically have lower numbers
(and were probably implemented earlier).

   There's no particular reason that it couldn't have been designed
with the initial FS tree as ID 0, but it simply wasn't. However,
changing this value now would result in two incompatible versions of
btrfs -- neither one would be able to deal with the other's
filesystems, because the FS tree has a different ID. (And writing code
to cope with both would be painful, disruptive and error-prone.)

   The cost of fixing this minor nit would, I think, far outweigh any
benefits you'd get from it. Hence the "alias" for 0, which is (IIRC)
done up-front in the ioctl interface, and therefore has few places
that it could go wrong or affect the main code of the FS.

   Hugo.

-- 
Hugo Mills             | "How deep will this sub go?"
hugo@... carfax.org.uk | "Oh, she'll go all the way to the bottom if we don't
http://carfax.org.uk/  | stop her."
PGP: 65E74AC0          |                                                  U571

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Re: root subvol id is 0 or 5?

[Duncan]

Hugo Mills posted on Sun, 30 Nov 2014 13:53:28 +0000 as excerpted:

> On Sun, Nov 30, 2014 at 07:08:51PM +0530, Shriramana Sharma wrote:
>> On Sun, Nov 30, 2014 at 5:29 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
>> >
>> >    In the data structures on disk, it's 5. The kernel aliases 0 to
>> > mean subvolid 5.
>> 
>> So why 5 and not just 0 which seems a logical choice? On top of this,
>> one needs to alias 0 to 5!
> 
>    All of the trees used in the FS metadata have an ID number. The
> "well-known" trees have small, fixed IDs:

Thanks, Hugo.

You might wish to find a place in the wiki (probably in the FAQ) for 
that, since your explanation was both the clearest I can imagine and 
cleared up some lingering "but why?" questions along that line for me, as 
well.

And if an answer to that basic a btrfs question is still clearing stuff 
up for me, I expect it could be useful to well over 90% of potential 
btrfs wiki FAQ readers...

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman