Description of problem:
When a DM thin-pool runs out of space it will automatically start queuing all outstanding IO destined to a thin device backed by that thin-pool.  This is similar to the dm-multipath "queue_if_no_path" option.

However, many times it is not desirable for the system to start queuing IO (potentially) indefinitely.  We do suggest the admin always have configured 1) a sane low water mark 2) enough free space in the VG that should the low water mark be hit, lvm2 will resize the volume.

BUT admins may _never_ want to queue IO if it comes to the fact that the thin-pool has actually ran out of space.

I discussed having the kernel manage transitioning a thin-pool from read-write access to read-only based on a low space threshold (likely one that is below the low water mark).  But Joe pointed out that there is no reason for this policy to be in the kernel.

Proposal is for lvm2 to:
   expose something like "read_only_if_low_space_threshold" in lvm.conf
   - if 100, then read_only transition is disabled.
   - if < 100, then once the free space reaches this percentage, switch to read-only


Additional info:

The existing kernel mechanism for controlling read_only vs read_write access is to use the "read_only" feature flag on thin-pool table load.

SO lvm2 could selectively reload the thin-pool table either with or without the "read_only" feature based on the lvm.conf "read_only_if_low_space_threshold".

BUT.. there is a dm-thin kernel bug/limitation that disallows transitioning a thin-pool table from RO back to RW via table reload.  This would obviously need to be fixed.