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Date: Sun, 30 Nov 2014 08:48:49 +0100
From: Goffredo Baroncelli <kreijack@inwind.it>
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To: Shriramana Sharma <samjnaa@gmail.com>,
        linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: root subvol id is 0 or 5?
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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;

[...]


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