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SSD TRIM software raid (mdadm)

I have create a software raid level 1 with mdadm.

One drive is a "classic" HDD.
The 2'nd drive is a SSD with option "write-mostly".

Over the raid I have create and LVM with all the partitions (root,swap
and qemu/KVM VM's).

When I understand mdadm the hole space is marked as used.
So my question is how useful is fstrim on /dev/mdx and would it relay
trim the SSD?

Best Regards,