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Date: Tue, 22 Aug 2017 21:48:18 +0500
From: Roman Mamedov <rm@romanrm.net>
To: Ulli Horlacher <framstag@rus.uni-stuttgart.de>
Cc: Linux fs Btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: finding root filesystem of a subvolume?
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On Tue, 22 Aug 2017 17:45:37 +0200
Ulli Horlacher <framstag@rus.uni-stuttgart.de> wrote:

> In perl I have now:
> 
> $root = $volume;
> while (`btrfs subvolume show "$root" 2>/dev/null` !~ /toplevel subvolume/) {
>   $root = dirname($root);
>   last if $root eq '/';
> }
> 
> 

If you are okay with rolling your own solutions like this, take a look at
"btrfs filesystem usage <path>". It will print the blockdevice used for
mounting the base FS. From that you can find the mountpoint via /proc/mounts.

Performance-wise it seems to work instantly on an almost full 2TB FS.

-- 
With respect,
Roman