Re: find subvolume directories

From: Andrei Borzenkov <arvidjaar@gmail.com>
To: linux-btrfs@vger.kernel.org
Subject: Re: find subvolume directories
Date: Sat, 13 Jul 2019 06:59:04 +0300	[thread overview]
Message-ID: <75e5bd20-fafa-07fd-afd9-159e9aacb7db@gmail.com> (raw)
In-Reply-To: <20190712231705.GA16856@tik.uni-stuttgart.de>

13.07.2019 2:17, Ulli Horlacher пишет:
> I need to find (all) subvolume directories.
> I know, btrfs subvolumes root directories have inode #256, but a
> "find / -inum 256" is horrible slow!
> 
> Next idea: "btrfs subvolume list /" is really fast, but its output is not
> always direct usable to find the subvolume directory.
> 
> Example output on a SUSE system:
> 
> root@trulla:~# btrfs subvolume list /
> ID 257 gen 2280099 top level 5 path @
> ID 258 gen 2280769 top level 257 path @/home
> ID 259 gen 2280947 top level 257 path @/opt
> ID 260 gen 2280098 top level 257 path @/srv
> ID 261 gen 2280954 top level 257 path @/tmp
> ID 262 gen 2280187 top level 257 path @/usr/local
> ID 263 gen 2280099 top level 257 path @/var/crash
> ID 264 gen 2280949 top level 257 path @/var/log
> ID 265 gen 2280099 top level 257 path @/var/opt
> ID 266 gen 2280954 top level 257 path @/var/spool
> ID 267 gen 2280947 top level 257 path @/var/tmp
> ID 270 gen 2280222 top level 257 path @/.snapshots
> ID 453 gen 2280954 top level 270 path @/.snapshots/128/snapshot
> ID 1235 gen 2280099 top level 257 path @/var/lib/machines
> ID 12392 gen 2123118 top level 270 path @/.snapshots/1065/snapshot
> ID 12393 gen 2123120 top level 270 path @/.snapshots/1066/snapshot
> ID 13273 gen 2176640 top level 270 path @/.snapshots/1089/snapshot
> ID 13274 gen 2176651 top level 270 path @/.snapshots/1090/snapshot
> ID 13553 gen 2203681 top level 270 path @/.snapshots/1103/snapshot
> 
> There is no /@/ directory in the default filesystem because of:
> 
> root@trulla:/# stat /@/.snapshots/128/snapshot
> stat: cannot stat '/@/.snapshots/128/snapshot': No such file or directory
> 
> root@trulla:~# btrfs subvolume get-default /
> ID 453 gen 2280954 top level 270 path @/.snapshots/128/snapshot
> 
> root@trulla:/# mount | grep " / "
> /dev/sda2 on / type btrfs (rw,relatime,space_cache,subvolid=453,subvol=/@/.snapshots/128/snapshot)
> 
> On this particular system I could remove "@" from the subvolume path to
> get the subvolume directory:
> 

That is just coincidence because @/.snapshot subvolume is mounted on
/.snapshot. It could also be mounted under /var/lib/snapper (insert your
path here).

> root@trulla:/# stat /.snapshots/128/snapshot
>   File: '/.snapshots/128/snapshot'
>   Size: 198             Blocks: 0          IO Block: 4096   directory
> Device: 27h/39d Inode: 256         Links: 1
> Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
> Access: 2019-07-13 01:03:08.543830085 +0200
> Modify: 2019-07-13 01:03:37.336445461 +0200
> Change: 2019-07-13 01:03:37.336445461 +0200
>  Birth: -
> 
> But what if a btrfs filesystem does not have a toplevel /@/ directory, but
> anything else, like /this/is/my/top/directory ?
> 

btrfs does not have "top level directory" beyond single /. It is
entirely up to the user who creates it how subvolumes are named and
structured. You can well have /foo, /bar, /baz mounted as /, /var and /home.

> Will be the first output line of "btrfs subvolume list /"
> always look like
> 
> ID 257 gen 2280099 top level 5 path this/is/my/top/directory 
> 
> ?
> 

If /foo and /foo/bar are subvolumes, then /foo/bar will have higher ID
than /foo (because it must have been created after /foo) so /foo will be
listed before /foo/bar in default sort order. Whether it will have ID
257 is entirely up to whoever created this filesytsem and its history.