feat(get): consolidate GET performance optimization (#3972)

* feat(get): consolidate GET performance optimization

Consolidated implementation of all GET performance optimizations into
a single, well-organized commit replacing the previous patch-on-patch
approach.

## Changes

### Configuration (set_disk/mod.rs)
- Consolidated all GET optimization flags into a single organized section
- Enabled by default: codec streaming, metadata early-stop, page cache reclaim
- Added codec streaming multipart flag (default: disabled)
- Added version-aware early-stop flag (default: disabled)
- Added adaptive duplex buffer sizing based on object size
- All flags use OnceLock caching with rollout percentage support

### Metadata Early-Stop (set_disk/read.rs)
- Delete marker early-stop when quorum agrees
- Version-aware early-stop for versioned GET requests
- MetadataQuorumAccumulator enhanced with:
  - delete_marker_votes tracking
  - requested_version_id and matching_version_votes tracking
  - version_early_stop_decision() method
- 6 new tests for version early-stop scenarios

### Codec Streaming (erasure/coding/decode_reader.rs)
- DualInFlight (2-stripe lookahead) enabled by default

### Decode Pipeline (erasure/coding/decode.rs)
- Stripe prefetch count configuration
- Bitrot-decode overlap configuration

### Disk Layer (disk/local.rs)
- O_DIRECT read configuration constants (preparation)

### Metrics (io-metrics/lib.rs)
- BytesPool acquisition/return metrics
- Metadata phase duration with early-stop label
- Total duration with reader_path label

### Diagnostics (diagnostics/)
- Early-stop reason constants
- Pool tier/outcome label constants

### Observability (.docker/observability/)
- 3 Grafana dashboards for GET optimization monitoring
- Prometheus alert rules (6 alerts: 3 critical, 3 warning)
- Updated README.md and README_ZH.md with usage docs

### Config (config/src/constants/runtime.rs)
- Page cache reclaim read enabled by default

## Environment Variables

| Variable | Default | Description |
|----------|---------|-------------|
| RUSTFS_GET_CODEC_STREAMING_ENABLE | true | Codec streaming base flag |
| RUSTFS_GET_CODEC_STREAMING_ROLLOUT_PCT | 100 | Codec streaming rollout % |
| RUSTFS_GET_CODEC_STREAMING_MULTIPART_ENABLE | false | Multipart codec streaming |
| RUSTFS_GET_METADATA_EARLY_STOP_ENABLE | true | Early-stop base flag |
| RUSTFS_GET_METADATA_EARLY_STOP_ROLLOUT_PCT | 100 | Early-stop rollout % |
| RUSTFS_GET_METADATA_VERSION_EARLY_STOP_ENABLE | false | Version-aware early-stop |
| RUSTFS_OBJECT_FILE_CACHE_RECLAIM_READ_ENABLE | true | Page cache reclaim |
| RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE | false | O_DIRECT (preparation) |
| RUSTFS_GET_DECODE_STRIPE_PREFETCH_COUNT | 1 | Stripe prefetch |
| RUSTFS_GET_BITROT_DECODE_OVERLAP_ENABLE | false | Bitrot-decode overlap |
| RUSTFS_GET_CODEC_STREAMING_MAX_INFLIGHT | 2 | DualInFlight stripes |

## Rollback

All optimizations can be disabled via environment variables:
RUSTFS_GET_CODEC_STREAMING_ENABLE=false
RUSTFS_GET_METADATA_EARLY_STOP_ENABLE=false
RUSTFS_OBJECT_FILE_CACHE_RECLAIM_READ_ENABLE=false

Co-Authored-By: heihutu <heihutu@gmail.com>

* test(get): add stress test scripts for GET optimization validation

- quick-validate-get-optimization.sh: Quick 5-minute validation
- stress-test-get-optimization.sh: Full 30+ minute stress test
- README-stress-test.md: Usage documentation

Co-Authored-By: heihutu <heihutu@gmail.com>

* test(ecstore): align file cache reclaim defaults

* chore(deps): update redis and erasure codec

* test(ecstore): align decode fill policy default

* test(ecstore): align metadata early-stop default

* fix(ecstore): keep metadata early stop opt-in

---------

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/disk/local.rs

@@ -84,6 +84,29 @@ const EVENT_DISK_LOCAL_ACCESS_FAILED: &str = "disk_local_access_failed";
 const EVENT_DISK_LOCAL_VOLUME_SETUP_FAILED: &str = "disk_local_volume_setup_failed";
 const EVENT_DISK_LOCAL_FORMAT_DECODE_FAILED: &str = "disk_local_format_decode_failed";
 
+/// Enable O_DIRECT for large sequential reads.
+/// When enabled, shard reads bypass the page cache using O_DIRECT flag.
+/// Requires aligned buffers (typically 512 bytes or 4096 bytes).
+/// Default: false (uses page cache via mmap/pread).
+const ENV_RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE: &str = "RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE";
+const DEFAULT_RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE: bool = false;
+
+/// Minimum shard size threshold for O_DIRECT reads.
+/// Only shards larger than this threshold will use O_DIRECT.
+/// Default: 4MB.
+const ENV_RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD: &str = "RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD";
+const DEFAULT_RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD: usize = 4 * 1024 * 1024;
+
+/// Check if O_DIRECT reads are enabled.
+fn is_direct_io_read_enabled() -> bool {
+    rustfs_utils::get_env_bool(ENV_RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE, DEFAULT_RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE)
+}
+
+/// Get the O_DIRECT read threshold size.
+fn get_direct_io_read_threshold() -> usize {
+    rustfs_utils::get_env_usize(ENV_RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD, DEFAULT_RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD)
+}
+
 fn log_startup_disk_io_error(stage: &str, path: &Path, err: &IoError) {
     warn!(
         event = EVENT_DISK_LOCAL_STARTUP_CLEANUP,
@@ -4982,7 +5005,16 @@ mod test {
     #[test]
     fn should_reclaim_file_cache_after_read_respects_env_and_threshold() {
         temp_env::with_var_unset(rustfs_config::ENV_OBJECT_FILE_CACHE_RECLAIM_READ_ENABLE, || {
-            assert!(!should_reclaim_file_cache_after_read(8 * 1024 * 1024));
+            temp_env::with_var_unset(rustfs_config::ENV_OBJECT_FILE_CACHE_RECLAIM_THRESHOLD, || {
+                assert!(should_reclaim_file_cache_after_read(8 * 1024 * 1024));
+                assert!(!should_reclaim_file_cache_after_read(1024));
+            });
+        });
+
+        temp_env::with_var(rustfs_config::ENV_OBJECT_FILE_CACHE_RECLAIM_READ_ENABLE, Some("false"), || {
+            temp_env::with_var(rustfs_config::ENV_OBJECT_FILE_CACHE_RECLAIM_THRESHOLD, Some("4194304"), || {
+                assert!(!should_reclaim_file_cache_after_read(8 * 1024 * 1024));
+            });
         });
 
         temp_env::with_var(rustfs_config::ENV_OBJECT_FILE_CACHE_RECLAIM_READ_ENABLE, Some("true"), || {