Change DEFAULT_FSYNC_BLOCKING_THREADS from 0 to 64 to isolate
fsync/fdatasync operations into a dedicated blocking thread pool.
A/B validation on 4-node EC cluster (testing, 10.0.0.5/8/9/11:9000):
PUT 256KiB c16: p99 226ms → 135ms (−40%), p50 60ms → 19ms (−68%)
GET 256KiB c16: p99 3.97ms → 3.63ms (−9%), throughput +1.8%
GET 4KiB c64: neutral (pure read, no fsync involvement)
Without isolation, fsync operations contend with read I/O (pread/stat/open)
on the main blocking pool, causing device-bound fsync to starve read
operations under mixed PUT+GET workloads.
Co-authored-by: heihutu <[email protected]>
