diff --git a/docs/operations/object-io-tuning-ab-matrix.md b/docs/operations/object-io-tuning-ab-matrix.md
new file mode 100644
index 000000000..baf0b906b
--- /dev/null
+++ b/docs/operations/object-io-tuning-ab-matrix.md
@@ -0,0 +1,218 @@
+# Object I/O (GET/PUT) tuning A/B matrix runbook
+
+> Scope: **parameter tuning** — measuring the effect of changing one
+> `RUSTFS_*` runtime knob at a time, against a fixed binary. This is
+> deliberately different from the code-change A/B gate in
+> [`hotpath-warp-ab-runbook.md`](hotpath-warp-ab-runbook.md) and the formal
+> ABBA validation in
+> [`hotpath-warp-abba-runbook.md`](hotpath-warp-abba-runbook.md), which compare
+> a baseline binary against a candidate binary.
+>
+> When a knob change turns out to need a code change, use those two runbooks
+> for the code-level validation and come back here for the knob-level sweep.
+
+## 1. What this runbook answers
+
+For each tuning knob it answers three questions:
+
+1. Which stage is actually slow — `set_disk_encode`, `set_disk_rename`,
+   `metadata_fanout`, `bitrot_verify`, etc.?
+2. Is the knob the real bottleneck, or is the stage slow for another reason?
+3. Does widening/loosening the knob buy throughput without an unacceptable
+   memory (RSS) or tail-latency regression?
+
+The core discipline is **one variable per A/B cell**. Never change two knobs in
+the same cell, or the result is unexplainable.
+
+## 2. Prerequisites
+
+- Linux bench host (or an ansible-managed cluster); a laptop smoke run is too
+  noisy to decide anything.
+- `warp` on `PATH` (or pass `--warp-bin` to the driver).
+- The observability metrics runtime **enabled**. The stage histograms below are
+  not emitted when `RUSTFS_OBS_METRICS_EXPORT_ENABLED=false` or the runtime is
+  otherwise off — see
+  [`hotpath-warp-ab-runbook.md`](hotpath-warp-ab-runbook.md) for the no-log /
+  no-monitor baseline env.
+- A warm, disposable data set. Recreate the bucket per run; do not bench against
+  production data.
+
+Load driver and gate are reused, not reimplemented:
+
+- `scripts/run_object_batch_bench_enhanced.sh` — warp driver with rounds,
+  median aggregation, `baseline_compare.csv`, and Prometheus service-metric
+  capture.
+- `scripts/hotpath_warp_ab_gate.sh` — relative budget gate over the deltas.
+- `scripts/run_hotpath_warp_ab.sh` — optional orchestrator when a knob needs
+  the full baseline-vs-candidate treatment (e.g. two different defaults).
+
+## 3. Fixed test conditions (lock before you start)
+
+Record these per run; a result without them is not reproducible:
+
+```text
+nodes, disks_per_node, total_disks, cpu_per_node, mem_per_node,
+network, erasure_set_drive_count, endpoint_mode (direct|lb),
+rustfs_commit_sha, warp --version, durability mode
+```
+
+Workload matrix (the same shapes the hotpath gate uses, expanded for the
+stage-breakdown object sizes):
+
+| Workload | mode | sizes |
+| --- | --- | --- |
+| small-fixed | put / get | 4KiB, 100KiB |
+| ec-boundary | put / get | 1MiB, 4MiB |
+| large-stream | put / get | 10MiB, 16MiB, 32MiB |
+| mixed | mixed | 256KiB |
+
+Concurrency ladder: `8, 16, 32, 64` (add `96, 128` on a bigger rig). Duration
+`120s`, `--rounds >= 3`, cooldown `>= 30s`.
+
+Isolate background noise before the sweep: scanner deep-verify, heal,
+replication, lifecycle transition, periodic capacity refresh — record whether
+each is on rather than silently assuming it is off.
+
+## 4. Measurement stack
+
+The code already instruments every stage below. Drive each A/B cell with these
+histograms (names verified against `crates/io-metrics/src/lib.rs`):
+
+- PUT stages: `rustfs_s3_put_object_stage_duration_ms{stage=...}` — compute
+  P50/P95/P99 per stage. Stages: `app_bucket_validate`, `app_sse_config_lookup`,
+  `app_object_lock_config_lookup`, `app_put_opts_build`, `app_prelookup`,
+  `ingress_prepare`, `app_encryption_prepare`, `app_replication_decision`,
+  `app_store_put`, `app_post_store_bookkeeping`, `app_capacity_update`,
+  `set_disk_writer_setup`, `set_disk_encode`, `set_disk_rename`,
+  `set_disk_old_data_cleanup`.
+- GET stages: `rustfs_io_get_object_stage_duration_seconds{path=..., stage=...}` —
+  the `path` label separates the read paths: `legacy_duplex`, `codec_streaming`,
+  `direct_memory`, `body_cache`, `inline_direct`, `internal_meta`,
+  `remote_transition`, `set_disk`, `empty`. Stages: `metadata`,
+  `metadata_cache_lookup`, `metadata_fanout`, `metadata_resolve`, `object_info`,
+  `path_decision`, `quorum_reached`, `range`, `reader_setup`,
+  `stripe_read`, `stripe_read_first_shard`, `stripe_read_quorum`, `decode`,
+  `reconstruct`, `emit`, `fill`, `output_poll`, `output_lock_wait`,
+  `bitrot_verify`, `first_byte`, `full_body`, `response_handoff`,
+  `lock_acquire`.
+- EC memory pressure: `rustfs_ec_encode_inflight_bytes_current` and the
+  allocator reclaim gauge; plus node RSS and CPU.
+
+Host telemetry (collect alongside every cell):
+
+```bash
+pidstat -durh 5 > telemetry/pidstat.txt &
+mpstat 5 > telemetry/mpstat.txt &
+iostat -xz 5 > telemetry/iostat.txt &
+```
+
+## 5. Tuning knob catalog
+
+Defaults are verified against `crates/config/src/constants/object.rs` and
+`crates/ecstore/src/erasure/coding/encode.rs`.
+
+### 5.1 PUT
+
+| Knob | Default | Controls | Validating stage | Risk if widened |
+| --- | --- | --- | --- | --- |
+| `RUSTFS_ERASURE_ENCODE_MAX_INFLIGHT_BYTES` | 32MiB | EC encode producer/consumer memory budget (blocks queued between encode and shard write) | `set_disk_encode` P95 + `rustfs_ec_encode_inflight_bytes_current` | RSS growth under high concurrency |
+| `RUSTFS_OBJECT_IO_BUFFER_SIZE` | 128KiB | Streaming read-in / write-out block size | `ingress_prepare`, `set_disk_encode` | Larger buffers = fewer polls, more resident memory |
+| `RUSTFS_OBJECT_DUPLEX_BUFFER_SIZE` | 4MiB | duplex pipe capacity (shared, but PUT path uses it less than GET) | `set_disk_encode` feed smoothness | Memory per in-flight request |
+| `RUSTFS_DURABILITY_MODE` / `RUSTFS_DRIVE_SYNC_ENABLE` | mode-dependent | per-shard fsync/sync discipline on commit | `set_disk_rename` P99 | Weakening it changes the durability contract — treat as a deliberate tradeoff, not a free win |
+| `RUSTFS_RUNTIME_WORKER_THREADS` / `RUSTFS_RUNTIME_MAX_BLOCKING_THREADS` | Tokio defaults | async workers + `spawn_blocking` pool feeding per-block encode | `set_disk_encode` P95 + mpstat | Oversubscription |
+
+### 5.2 GET
+
+| Knob | Default | Controls | Validating stage | Risk if enabled |
+| --- | --- | --- | --- | --- |
+| `RUSTFS_GET_CODEC_STREAMING_ROLLOUT` | `off` | switches the read path from `legacy_duplex` to the pull-based `ErasureDecodeReader` (`codec_streaming`) | compare `path="legacy_duplex"` vs `path="codec_streaming"` for `decode`/`emit`/`output_lock_wait`/`stripe_read` | behavioral change to the read path; rollout is `off` by default for a reason |
+| `RUSTFS_GET_CODEC_STREAMING_ENGINE` | `legacy` | `legacy` vs `rustfs` decode engine under the streaming reader | `reconstruct`/`decode` per `path` | engine swap on a correctness-critical path |
+| `RUSTFS_GET_CODEC_STREAMING_MULTIPART_ENABLE` | `false` | multipart objects on the streaming reader | same, multipart cells | wider format coverage |
+| `RUSTFS_GET_CODEC_STREAMING_DATA_BLOCKS_FIRST_ENABLE` (+ `_MAX_SIZE`, `_FIRST_READER_SETUP`) | `false` / 512KiB | prefer data-shard readers before parity | `stripe_read_first_shard`/`stripe_read_quorum` | shard-selection order change |
+| `RUSTFS_OBJECT_GET_SKIP_BITROT_VERIFY` | `false` | skip per-shard HighwayHash verify | `bitrot_verify` | **do not default on** — measures the theoretical ceiling only |
+| `RUSTFS_OBJECT_DUPLEX_BUFFER_SIZE` | 4MiB | legacy GET in-process pipe capacity | `output_lock_wait`/`output_poll` | memory per in-flight GET |
+| `RUSTFS_GET_SEEK_BUFFER_ENABLE` | `false` | in-memory seek buffer for small GET | `first_byte` | experimental, startup-latched — see [`get-path-experimental-switches.md`](get-path-experimental-switches.md) |
+| `RUSTFS_GET_OUTPUT_HANDOFF_ATTRIBUTION_ENABLE` | `false` | adds `response_handoff` attribution (metrics only) | `response_handoff` | small per-request bookkeeping cost |
+
+## 6. A/B matrix
+
+Run each row as an independent cell. Baseline is the shipped default; candidate
+is one knob moved. Everything else (topology, sizes, concurrency, rounds,
+durability) stays fixed.
+
+### 6.1 PUT
+
+| # | Knob | Baseline | Candidates | Signal metric | Judgement |
+| --- | --- | --- | --- | --- | --- |
+| P1 | encode in-flight | 32MiB | 48MiB, 64MiB, 96MiB | `set_disk_encode` P95 + throughput + RSS | throughput up AND `set_disk_encode` down AND RSS tolerable → budget was the binding constraint |
+| P2 | object I/O buffer | 128KiB | 256KiB, 512KiB, 1MiB | `ingress_prepare` + `set_disk_encode` | encode smooths with bounded RSS → upstream feed was too small |
+| P3 | duplex buffer | 4MiB | 8MiB, 16MiB | `set_disk_encode` feed variance | lower priority than P1/P2 |
+| P4 | blocking threads | default | 512, 768, 1024 | `set_disk_encode` P95 + mpstat | per-block `spawn_blocking` is scheduler-bound if P95 falls |
+| P5 | durability | current mode | weaker/stronger mode | `set_disk_rename` P99 | only as a **deliberate durability tradeoff**, never a silent default change |
+| P6 | set drive count | current | other valid widths | all PUT stages | highest cost — only after P1–P5, and only on a rebuildable topology |
+
+### 6.2 GET
+
+| # | Knob | Baseline | Candidates | Signal metric | Judgement |
+| --- | --- | --- | --- | --- | --- |
+| G1 | codec streaming rollout | `off` | `on` (pct ramp 10/50/100) | `path="legacy_duplex"` vs `path="codec_streaming"` for `decode`/`emit`/`output_lock_wait`/`stripe_read` + throughput | streaming beats duplex on `decode`+`output_lock_wait` and byte-for-byte output matches → candidate for default |
+| G2 | codec streaming engine | `legacy` | `rustfs` | `reconstruct`/`decode` per `path` | engine swap is neutral-or-better on CPU with identical bytes |
+| G3 | multipart streaming | `false` | `true` | multipart GET cells | only after G1 is stable on single-part |
+| G4 | data-blocks-first | `false` | `true` | `stripe_read_first_shard`/`stripe_read_quorum` | fewer shard reads without a correctness regression |
+| G5 | duplex buffer | 4MiB | 8MiB, 16MiB | `output_lock_wait`/`output_poll` (legacy path) | only if still on `legacy_duplex` |
+| G6 | skip bitrot verify | `false` | `true` | `bitrot_verify` | **ceiling measurement only**; do not carry into production |
+
+## 7. Execution sequence
+
+1. Freeze the conditions in §3 and record the provenance block.
+2. Run the baseline cell (all defaults) and capture stage histograms + host
+   telemetry.
+3. Pick the **one** most-likely knob from the analysis. For large-object PUT
+   that is almost always `set_disk_encode` → P1; for GET it is G1 (the
+   `legacy_duplex` → `codec_streaming` switch).
+4. Sweep that knob's candidate column one value at a time, same workload.
+5. Read the decision table in §8; if the stage did not move, the knob is not
+   the bottleneck — stop widening it and pick the next stage.
+
+Driver invocation for one PUT cell:
+
+```bash
+scripts/run_object_batch_bench_enhanced.sh \
+  --tool warp --endpoint http://127.0.0.1:9000 \
+  --access-key "$RUSTFS_ACCESS_KEY" --secret-key "$RUSTFS_SECRET_KEY" \
+  --bucket rustfs-put-tuning --warp-mode put \
+  --sizes 16MiB,32MiB --concurrency 32 --duration 120s --rounds 3 \
+  --out-dir target/bench/put-tuning-p1-64mib
+```
+
+## 8. Interpretation / decision table
+
+| Stage high | Most likely cause | Next action |
+| --- | --- | --- |
+| `set_disk_encode` | per-block EC encode scheduling + in-flight budget | P1 → P4 → P2, in that order |
+| `set_disk_rename` | commit tail (rename fan-out / RPC / fsync) | P5 (durability) and cluster tail analysis, not encode |
+| `set_disk_writer_setup` | per-disk `BitrotWriter` + temp-file create | disk/filesystem metadata; per-disk fan-out cost |
+| `set_disk_old_data_cleanup` | overwrite / versioned-object directory delete | confirm overwrite-vs-new-write; defer cleanup further |
+| `metadata_fanout` / `metadata_resolve` | cross-disk `xl.meta` read + quorum | metadata cache hit rate; small-object fixed cost |
+| `bitrot_verify` | HighwayHash verify on the read path | G6 ceiling only; do not default on |
+| `output_lock_wait` / `output_poll` | legacy duplex backpressure | G1 (move off duplex) or G5 |
+| `stripe_read*` | shard concurrency / selection | G4 shard-selection, disk/network tail |
+
+## 9. Guardrails
+
+- **Never weaken correctness for throughput**: read/write quorum, bitrot verify,
+  `xl.meta` validation, and durability (`RUSTFS_DURABILITY_MODE`) are integrity
+  contracts, not knobs. P5 and G6 are ceiling measurements and must be labelled
+  as such; do not carry their values into production without an explicit
+  durability/correctness decision.
+- **One variable per cell.** A cell that changes two knobs is thrown away.
+- **Memory is part of the result.** A throughput win with unbounded RSS growth
+  is a regression; record RSS and the EC in-flight gauge for every PUT cell.
+- **Startup-latched knobs** (`RUSTFS_GET_SEEK_BUFFER_ENABLE`,
+  `RUSTFS_GET_OUTPUT_HANDOFF_ATTRIBUTION_ENABLE`, and the codec-streaming
+  switches) require a process restart to change — see
+  [`get-path-experimental-switches.md`](get-path-experimental-switches.md).
+- **Archive the raw data.** Keep the `baseline_compare.csv`, `median_summary.csv`,
+  stage histograms, and host telemetry per cell; the conclusion must trace back
+  to them. Do not commit benchmark result snapshots to the repo — record them in
+  the issue tracker.
diff --git a/rustfs/src/app/object_usecase.rs b/rustfs/src/app/object_usecase.rs
index 3470b48d8..d404add26 100644
--- a/rustfs/src/app/object_usecase.rs
+++ b/rustfs/src/app/object_usecase.rs
@@ -134,7 +134,6 @@ use rustfs_notify::EventArgsBuilder;
 use rustfs_object_capacity::capacity_manager::get_capacity_manager;
 use rustfs_policy::policy::action::{Action, S3Action};
 use rustfs_s3_ops::{S3Operation, delete_event_name_for_marker, put_event_name_for_post_object};
-use rustfs_s3select_api::object_store::bytes_stream;
 use rustfs_targets::{EventName, get_request_host, get_request_port, get_request_user_agent};
 use rustfs_utils::CompressionAlgorithm;
 #[cfg(test)]
@@ -918,7 +917,7 @@ pin_project! {
 }
 
 struct MemoryTrackedBytesStream {
-    bytes: Bytes,
+    bytes: Option<Bytes>,
     emitted: bool,
     completed: bool,
     expected: usize,
@@ -1032,7 +1031,7 @@ impl MemoryTrackedBytesStream {
     ) -> Self {
         let length_mismatch = bytes.len() != expected;
         Self {
-            bytes,
+            bytes: Some(bytes),
             emitted: false,
             completed: !length_mismatch && expected == 0,
             expected,
@@ -1100,35 +1099,36 @@ impl futures::Stream for MemoryTrackedBytesStream {
         // differently sized body. This is a defense-in-depth backstop; the
         // buffered/cache callers reject the mismatch before headers are sent.
         if this.length_mismatch {
+            let actual = this.bytes.as_ref().map_or(0, Bytes::len);
             this.emitted = true;
             this.finish_err();
             return Poll::Ready(Some(Err(std::io::Error::new(
                 std::io::ErrorKind::InvalidData,
-                format!(
-                    "materialized GET body length mismatch: expected {}, got {}",
-                    this.expected,
-                    this.bytes.len()
-                ),
+                format!("materialized GET body length mismatch: expected {}, got {}", this.expected, actual),
             ))));
         }
 
-        let first_byte_elapsed = (!this.bytes.is_empty()).then(|| this.started.elapsed());
+        let Some(bytes) = this.bytes.take() else {
+            return Poll::Ready(None);
+        };
+        let bytes_len = bytes.len();
+        let first_byte_elapsed = (!bytes.is_empty()).then(|| this.started.elapsed());
         this.emitted = true;
         if let Some(elapsed) = first_byte_elapsed {
             rustfs_io_metrics::record_get_object_first_byte_latency(GET_OBJECT_STAGE_PATH_S3_HANDLER, elapsed.as_secs_f64());
         }
-        if this.bytes.len() >= this.expected {
+        if bytes_len >= this.expected {
             this.finish_ok();
         }
         if let Some(poll_start) = poll_start {
             rustfs_io_metrics::record_get_object_memory_body_stream_poll(
                 this.source,
                 GET_READER_STREAM_POLL_READY_DATA,
-                this.bytes.len(),
+                bytes_len,
                 poll_start.elapsed().as_secs_f64(),
             );
         }
-        Poll::Ready(Some(Ok(this.bytes.clone())))
+        Poll::Ready(Some(Ok(bytes)))
     }
 }
 
@@ -4148,10 +4148,7 @@ impl DefaultObjectUsecase {
         let bytes_len = bytes.len();
         let guard = rustfs_io_metrics::track_get_object_buffered_bytes(bytes_len);
         let remaining = usize::try_from(response_content_length.max(0)).unwrap_or(usize::MAX);
-        let blob = StreamingBlob::wrap(bytes_stream(
-            MemoryTrackedBytesStream::new(bytes, remaining, source, guard, lifecycle),
-            remaining,
-        ));
+        let blob = StreamingBlob::wrap(MemoryTrackedBytesStream::new(bytes, remaining, source, guard, lifecycle));
         if let Some(handoff_start) = handoff_start {
             rustfs_io_metrics::record_get_object_response_handoff(
                 "single_chunk",