K 10
svn:author
V 3
mav
K 8
svn:date
V 27
2019-02-12T00:53:43.496550Z
K 7
svn:log
V 728
MFC r343562, r343563: Reimplement BIO_ORDERED handling in nvd(4).

This fixes BIO_ORDERED semantics while also improving performance by:
 - sleeping also before BIO_ORDERED bio, as defined, not only after;
 - not queueing BIO_ORDERED bio to taskqueue if no other bios running;
 - waking up sleeping taskqueue explicitly rather then rely on polling.

On Samsung SSD 970 PRO this shows sync write latency, measured with
`diskinfo -wS`, reduction from ~2ms to ~1.1ms by not sleeping without
reason till next HZ tick.

On the same device ZFS pool with 8 ZVOLs synchronously writing 4KB blocks
shows ~950 IOPS instead of ~750 IOPS before.  I suspect ZFS does not need
BIO_ORDERED on BIO_FLUSH at all, but that will be next question.

END