Unusual ZFS behaviour

Hi,

I'm running a somewhat recent version of STABLE-13/amd64: 
stable/13-n256681-0b7939d725ba: Fri Nov 10 08:48:36 NZDT 2023, and I'm 
seeing some unusual behaviour with ZFS.

To reproduce:
  1. one big empty disk, GPT scheme, 1 freebsd-zfs partition.
  2. create a zpool, eg: tank
  3. create 2 sub-filesystems, eg: tank/one, tank/two
  4. fill each sub-filesystem with large files until the pool is ~80% 
full. In my case I had 200 10Gb files in each.
  5. in one session run 'md5 tank/one/*'
  6. in another session run 'md5 tank/two/*'

For most of my runs, one of the sessions against a sub-filesystem will 
be starved of I/O, while the other one is performant.

Is anyone else seeing this?
-- 
Jonathan Chen <jonc@chen.org.nz>