MFC r352939: Improve latency of synchronous 128KB writes.
Before my ZIL space optimization few years ago 128KB writes were logged
as two 64KB+ records in two 128KB log blocks. After that change it became
~124KB+/4KB+ in two 128KB log blocks to free space in the second block
for another record. Unfortunately in case of 128KB only writes, when space
in the second block remained unused, that change increased write latency by
imbalancing checksum computation time between parallel threads.
This change introduces new 68KB log block size, used for both writes below
67KB and 128KB-sharp writes. Writes of 68-127KB are still using one 128KB
block to not increase processing overhead. Writes above 131KB are still
using full 128KB blocks, since possible saving there is small. Mixed loads
will likely also fall back to previous 128KB, since code uses maximum of
the last 10 requested block sizes.
On a simple 128KB write test with queue depth of 1 this change demonstrates
~15-20% performance improvement.