Description of problem:
TRIM requests are not passed down to the underlying MD raid devices. Even with modern SSDs, like the Vertex 4, we still see slowdowns in workstation usage after available sectors are filled up.

Version-Release number of selected component (if applicable):
NA

How reproducible:
Always.

Steps to Reproduce:
1. Create a mirrored md array
2. Create a partition and ext4 filesystem (let's call it /mnt/mirror) on the md array
3. fstrim /mnt/mirror
  
Actual results:
fstrim: /mnt/mirror: FITRIM ioctl failed: Operation not supported

Expected results:
No error.

Additional info:
As I understand, TRIM support for MDRAID is not yet merged in the kernel, but patches do exist.