[Bug 209571] NVMe performing poorly. TRIM requests stall I/O activity

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209571

--- Comment #8 from Warner Losh <imp at FreeBSD.org> ---
Only if you are using nda, which most people aren't. It doesn't exist in 10 or
11 (well, until last week). It's not turned on by default in 12.

So talking about cam_ioschded and nvd in the same breath misunderstands the
problem. nvd doesn't collapse trims and that's the problem there. It doesn't
use CAM at all so no tweaks to cam_iosched is going to help because it's
completely unused. nda, on the other hand, has the collapsing trim issue and
the priority inversion issue cem points out. I've been tweaking the code to
reduce trim priority as well as collapse trims, but that won't help nvd. I have
no plans on doing anything with nvd.

So if you are using nvd, the whole problem is lack of trim collapsing. If you
are using nda, that problem is compounded by the I/O scheduler. I should have a
fix for nda by the end of next week.

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