In theory, Xen Orchestra got a protection against this issue (see https://xen-orchestra.com/docs/backup_troubleshooting.html#vdi-chain-protection )
So you have to find what's causing a long chain outside XO.
You can check in XO, in the Advanced tab of the SR view, if there's VDI to coalesce.
Or maybe very long chain for other reasons, there's a tool we made called xapi-explore-sr.