Hi!

A dm-era device created over a RAID5 array loses discard support.

Test script is attached.

If you run it on SSD you'll see something like:

loop0 4096
loop1 4096
loop2 4096
loop3 4096
md55 1048576
loop_test_era 0

I.e. mdadm device supports discard, but dm-era device started over it doesn't.

If you create a dm-era device over a normal block device without mdadm RAID5 or 
even over mdadm RAID1 - it will preserve discard support.

Why? How can it be fixed?

-- 
With best regards,
  Vitaliy Filippov

Attachment: raid5_test.sh
Description: Binary data

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