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Date:   Tue, 20 Jul 2021 18:10:44 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Josef Bacik <josef@toxicpanda.com>
Cc:     Christoph Hellwig <hch@infradead.org>, NeilBrown <neilb@suse.de>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>, linux-nfs@vger.kernel.org,
        Wang Yugui <wangyugui@e16-tech.com>,
        Ulli Horlacher <framstag@rus.uni-stuttgart.de>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.
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On Thu, Jul 15, 2021 at 02:01:11PM -0400, Josef Bacik wrote:
> The problem I ran into was the automount stuff requires that we have a
> completely different superblock for every vfsmount. This is fine for
> things like nfs or samba where the automount literally points to a
> completely different mount, but doesn't work for btrfs where it's on
> the same file system.  If you have 1000 subvolumes and run sync()
> you're going to write the superblock 1000 times for the same file
> system.

Dumb question: why do you have to write the superblock 1000 times, and
why is that slower than writing to 1000 different filesystems?

> You are
> going to reclaim inodes on the same file system 1000 times.  You are
> going to reclaim dcache on the same filesytem 1000 times.  You are
> also going to pin 1000 dentries/inodes into memory whenever you
> wander into these things because the super is going to hold them
> open.

That last part at least is the same for the 1000-different-filesystems
case, isn't it?

--b.

> This is not a workable solution.  It's not a matter of simply tying
> into existing infrastructure, we'd have to completely rework how the
> VFS deals with this stuff in order to be reasonable.  And when I
> brought this up to Al he told me I was insane and we absolutely had
> to have a different SB for every vfsmount, which means we can't use
> vfsmount for this, which means we don't have any other options.
> Thanks,
> 
> Josef