I see this in my dmesg log
EXT4-fs (md1): re-mounted. Opts: commit=0 EXT4-fs (md2): re-mounted. Opts: commit=0 EXT4-fs (md3): re-mounted. Opts: commit=0
I think that means that dealloc is disabled? does mdadm not support dealloc?
mdadm supports dealloc.
So I don't get the link between mdadm and commit=0 in your question?
commit=nrsec Sync all data and metadata every nrsec seconds. The default value is 5 seconds. Zero means default.
New drives unload heads. Would then need base on commit size and run the risk of loosing 10 min of work to save wear and tear on disc.