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