Unless I'm misinterpreting something it appears that maybe btrfs doesn't pass
fstrim commands down to the underlying drives when being used in a RAID-1 

I have this output from a small script I wrote to run at boot time (and also via
cron.weekly), rather than using continous trim in the boot options:

# cat /var/log/trim.log
Thu Oct 13 07:40:07 CDT 2016
/boot: 454 MiB (476062720 bytes) trimmed

Thu Oct 13 07:40:08 CDT 2016
/: 8.9 GiB (9585152000 bytes) trimmed

Thu Oct 13 07:40:22 CDT 2016
/btrfs/0: 0 B (0 bytes) trimmed

/boot and / are mdraid RAID 1 on partitions 1 and 3 of two Samsung 850 Pro SSDs.
/btrfs/0 is a btrfs-raid RAID 1 of partition 4 on the same two drives. The btrfs
case does not seem to accomplish anything. By comparison, I have the same drive
in my laptop, but just a single one, and the non-btrfs-raid-1 file system on one
of its partitions does run fstrim successfully.

This is quite possibly a known limitation, but I didn't find anything about it
through some quick searching. Maybe I didn't dive deep enough...

twalb...@gmail.com, twalb...@comcast.net
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