Bug#987069: document which file systems support cgroupv2 I/O controller

[Nicholas D Steeves <nsteeves@gmail.com>]

Package: release-notes
Severity: normal

Hi,

Last I checked, only btrfs supports the cgroupv2 I/O controller; this
should probably be documented.  Alternatively, if more than btrfs (ie:
XFS and ext4) supports it, but not other file systems (ie: f2fs,
reiser4, jfs, etc) than this should be documented.

As far as I can tell, the significance of this is as follows: Buster
switched to mq-deadline, which does not support the idle I/O priority,
and to get functional ionice the user/sysadmin had to switch to the
non-mq CFQ, or bfq.  The situation in a default Bullseye install is
better, but only for file systems that support the cgroupv2 I/O
controller.

Regards,
Nicholas