diff --git a/src/io/writeback_file/mod.rs b/src/io/writeback_file/mod.rs
index 424d4ff..b77bc3e 100644
--- a/src/io/writeback_file/mod.rs
+++ b/src/io/writeback_file/mod.rs
@@ -73,9 +73,14 @@ use std::path::Path;
 use super::writeback::WritebackPipeline;
 
 /// Granularity at which the Linux writeback pipeline issues
-/// `sync_file_range` / `posix_fadvise(DONTNEED)` pairs. 32 MiB
-/// remains the best-tested value.
-const WRITEBACK_CHUNK_BYTES: u64 = 32 * 1024 * 1024;
+/// `sync_file_range` / `posix_fadvise(DONTNEED)` pairs.
+///
+/// iter9 (2026-05-17): 32 → 64 MiB on top of iter8 (Phase 2.5
+/// disabled). Without Phase 2.5's writer thread, WAIT_AFTER blocks
+/// the mux thread directly. Bigger chunks = fewer WAIT_AFTER calls
+/// = less mux-thread blocking. iter8 mean was 28.7 with 32 MiB; aim
+/// to clear the 30 floor by halving WAIT_AFTER frequency.
+const WRITEBACK_CHUNK_BYTES: u64 = 64 * 1024 * 1024;
 
 pub(crate) struct WritebackFile {
     file: File,