Concluding from NTFS, defrag will run in the background, at low priority, not harming any data.
IO overload is IO overload.
How much data has the node? is the 10% leftover free?
I set mine temporarily to full when i defrag them while running, so they only perform reads and deletes. For like a week at 6TB data.
Maybe you can ofload databases to an other physicaly drive.
i only know it for windows
even some usb sticks are enough for that. mine run on Samsung BAR Plus USB-Stick Typ-A, 128 GB, since august. because i did not want to bother the system ssd.
(just format one with an suitable filesystem and low clustersize eg for me ntfs,4k for you ext4)
even smaller one will go temporarily. you can move them back later, if you want.
today i saw an 128gb ssd for 9,49€ (was no option for me since i had a BRIX)