[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

Conrad Meyer <cem at freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |cem at freebsd.org
            Summary|ZFS and NVMe performing     |NVMe performing poorly.
                   |poorly. TRIM requests stall |TRIM requests stall I/O
                   |I/O activity                |activity

--- Comment #6 from Conrad Meyer <cem at freebsd.org> ---
I see the exact same problem with TRIM, Samsung 960 EVO, and UFS.

Part of the problem is consumer controller (960 EVO) doing a bad job with TRIM.

Part of the problem is nvd does not coalesces TRIMs.  Part of the problem is
cam_iosched separates out and prioritizes TRIM over all other IO; see
cam_iosched_next_bio().  (Separating out is useful for coalescing, but we don't
actually coalesce yet.)

I observed iostat showing non-zero queue depths and long IO latencies with TRIM
enabled (960 EVO).  With TRIM disabled, qdepth was at most ever 1 and IO
latency fell drastically under the same workload.

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