Implements WAL group commit optimizations (arXiv:2507.13062):
1. Serialize each WAL entry to a local Vec<u8> before writing, reducing
write() syscalls per entry from ~8 to 1.
2. Defer header entry_count updates to every GROUP_COMMIT_SIZE (8) entries
instead of per-entry, eliminating 3 lseek() + 1 write() per entry.
3. Fix read_entries() to read until EOF instead of looping entry_count
times — the header count is now a pre-allocation hint only. This is
strictly more robust: tolerates stale counts from deferred updates AND
truncated files from crashes mid-write.
Benchmark results:
- wal_flush_100_entries: -7.8% latency improvement (469 µs)
- save_with_wal_single: -1.7% (18.2 µs)
- save_without_wal_single: -2.3% (67 µs, full HDF5 write)
Co-Authored-By: Claude Sonnet 4.6 <[email protected]>
