Re: [Findutils-patches] [PATCH] find memory leak

[Bernhard Voelker]

On 02/08/2017 07:03 PM, Goffredo Baroncelli wrote:
> For sure it is a btrfs bug (see the link which I posted in my other reply).
> However this is not a "design bug". There is a rationale behind this choice.
> 
> Because btrfs is capable to writable snapshot, it is possible that in the 
> same filesystem two files have the same inode-number even tough these 
> are/became different.
> So btrfs has to give to every subvolume a different device-id, to 
> differentiate files from the original one to the snapshot one.

sorry, IMO it /is/ a design bug if all tools relying on st_dev have
to add file system specific code just because that file system
consciously breaks the long-known st_dev <-> mountinfo relation.
Even latest df(1) does not show the fstype for btfs snapshots
(and forget about correct bftrfs file system usage numbers):

  $ ~/coreutils/src/df -Th /mnt /mnt/.snapshot /mnt/_subvol
  Filesystem     Type   Size  Used Avail Use% Mounted on
  /dev/loop1     btrfs   20G  429M   18G   3% /mnt
  -              -       20G  429M   18G   3% /mnt/.snapshot
  -              -       20G  429M   18G   3% /mnt/_subvol

After all, it is not (only) a performance issue for find(1), but a
general one for the -fstype option for btfs subvolumes and snapshots.

  $ find /mnt/_subvol | wc -l
  2

  $ find /mnt/_subvol -fstype btrfs -print | wc -l
  0

I don't know more about btrfs and this may sound like a crazy idea,
but it would maybe help quite some tools if btrfs would implicitly
add an entry to mountinfo for each subvolume/snapshot to avoid such
virtual device numbers ... which works e.g. if the admin creates a
bind mount for the subvolume to itself (example cont'd):

  $ mount --bind /mnt/_subvol /mnt/_subvol
  $ mount --bind /mnt/.snapshot /mnt/.snapshot

  $ find /mnt/_subvol -fstype btrfs -print | wc -l
  1

  $ ~/coreutils/src/df -Th /mnt /mnt/.snapshot /mnt/_subvol
  Filesystem     Type   Size  Used Avail Use% Mounted on
  /dev/loop1     btrfs   20G  429M   18G   3% /mnt
  /dev/loop1     btrfs   20G  429M   18G   3% /mnt/.snapshot
  /dev/loop1     btrfs   20G  429M   18G   3% /mnt/_subvol

Have a nice day,
Berny