Add SSD Trim Support

[giganode]

Hey there!

Firstly, I'd like to thank you for making it available outside Unraid. I wonder what you are thinking about adding Trim Support for SSD Arrays. As you know, with normal usage, the array slows down as more and more files get deleted.

I know, it is deactivated on purpose by limetech, because the driver doesn't recognize Trim as an event to trigger parity calculations.

[qvr]

I haven't really looked into it, but yes, the driver disables TRIM (discard) support explicitly:
https://github.com/qvr/nonraid/blob/main/md_nonraid/6.12/md_unraid.c#L1044

I suppose this originates from the original md raid driver, although apparently in there it is possible to enable discard support if you are certain your SSD handles discard correctly and will return zeroes when those discarded blocks are read again:
https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/dm-raid.html#discard-support

Quickly browsing the mdraid5 code it seems support for discard operations would need to be added to basically every low-level IO operation:
https://github.com/torvalds/linux/blob/master/drivers/md/raid5.c#L5684

So yeah, it's not just about enabling trim support, there's no code to handle the actual discard operations. And I don't have any plans to make major feature changes into the driver, which I think this would definitely be, but if someone really wants to take a stab at it, PRs would be welcome.