mixing raid1 and raid0?

[Eric Wong <e@80x24.org>]

Hey all, I've started using btrfs recently with old HDDs of
various sizes in a btrfs raid1 pool.  It seems pretty good from
a flexibility and redundancy standpoint.

However, things like temporary files, caches, etc. don't need to
be raid1, and raid0 might allow me to reduce wear on HDDs; so
I'd like to make part of it raid0 while keeping most of it raid1.

If my btrfs is mounted as /mnt/btrfs, would the following do
what I want?

  btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt/btrfs/precious
  btrfs balance start -dconvert=raid0 -mconvert=raid0 /mnt/btrfs/junk

Or should I make a separate FS for temporary and disposable
raid0 data?

Thanks