OpenZFS 9076 - Adjust perf test concurrency settings

ZFS Performance test concurrency should be lowered for better latency

Work by Stephen Blinick.

Nightly performance runs typically consist of two levels of concurrency;
and both are fairly high.

Since the IO runs are to a ZFS filesystem, within a zpool, which is
based on some variable number of vdev's, the amount of IO driven to each
device is variable. Additionally, different device types (HDD vs SSD,
etc) can generally handle a different amount of concurrent IO before
saturating.

Nevertheless, in practice, it appears that most tests are well past the
concurrency saturation point and therefore both perform with the same
throughput, the maximum of the device. Because the queuedepth to the
device(s) is so high however, the latency is much higher than the best
possible at that throughput, and increases linearly with the increase in
concurrency.

This means that changes in code that impact latency during normal
operation (before saturation) may not be apparent when a large component
of the measured latency is from the IO sitting in a queue to be
serviced. Therefore, changing the concurrency settings is recommended

Authored by: Stephen Blinick <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: John Wren Kennedy <[email protected]>
Ported-by: John Wren Kennedy <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/9076
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/562
Upstream bug: DLPX-45477
Closes #7302

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